Simple Rhubarb Lunar Cake

I’d never heard of a lunar cake until my neighbor brought one to our block party last spring. She called it that because of the way the cream cheese topping settles into little craters as it bakes—kind of like the surface of the moon.

The best part? It’s basically a coffee cake that you can throw together in one bowl. No mixer needed, no fancy technique required. Just cream your butter and sugars with a wooden spoon, add your wet ingredients, fold in the dry stuff, and you’re done. The rhubarb gets tossed right into the batter, and that cinnamon-walnut topping goes on last. It bakes up into this soft, slightly tangy cake that’s perfect with your morning coffee or as an after-dinner treat.

What Kind of Rhubarb Should I Use?

Fresh rhubarb is your best bet for this cake, and you’ll find it in season during spring and early summer at most grocery stores and farmers markets. The color of the stalks doesn’t really matter – whether you grab bright red, pink, or even greenish stalks, they’ll all taste pretty similar and work just fine in baking. Just make sure to trim off and discard the leaves completely, as they’re toxic and should never be eaten. If fresh rhubarb isn’t available, you can use frozen rhubarb instead – just don’t bother thawing it first, as it can get mushy, and toss it right into your batter while still frozen.

Options for Substitutions

This cake is pretty forgiving when it comes to swapping ingredients:

• Rhubarb: Fresh rhubarb is best for this recipe, but if you only have frozen, thaw it completely and drain any excess liquid before using. You can also mix rhubarb with strawberries for a sweeter flavor – use about 1 cup rhubarb and 1/2 cup strawberries.
• Buttermilk: No buttermilk? Make your own by mixing 1/2 cup regular milk with 1 1/2 teaspoons of lemon juice or white vinegar. Let it sit for 5 minutes before adding to your batter.
• Walnuts: Feel free to use pecans, almonds, or leave the nuts out entirely if you prefer. You could also swap them for rolled oats for a different texture on top.
• Butter: You can use coconut oil or vegetable oil instead, though butter gives the best flavor. If using oil, use the same amount.
• Brown sugar: Regular white sugar works fine if that’s all you have. The brown sugar adds a bit of molasses flavor, but the cake will still turn out great with white sugar.

Watch Out for These Mistakes While Baking

The biggest mistake when baking with rhubarb is not cutting it into uniform pieces – aim for 1/2-inch chunks so they cook evenly and don’t release too much liquid in some spots while staying tough in others.

Many bakers also forget that rhubarb is incredibly tart, so if your stalks are particularly sour, you might want to toss them with a tablespoon of sugar before mixing them into the batter to balance the flavors.

Don’t skip greasing your pan well, as the sugary topping can stick like crazy, and make sure to check for doneness by inserting a toothpick into the center – it should come out with just a few moist crumbs, not wet batter.

Finally, resist the urge to cut into this cake right away, since letting it cool for the full 10-15 minutes helps it set properly and makes slicing much cleaner.

What to Serve With Rhubarb Lunar Cake?

This cake is perfect on its own with a cup of coffee or tea for an afternoon snack, but a dollop of whipped cream or vanilla ice cream takes it to the next level. The tangy rhubarb pairs really well with something creamy and sweet, so even a simple spoonful of Greek yogurt works great if you want to keep things lighter. I love serving this cake warm from the oven so the ice cream melts into all the nooks and crannies. For a fancier presentation, you can drizzle some caramel sauce over the top or dust it with powdered sugar before serving.

Storage Instructions

Store: Keep your rhubarb lunar cake covered with plastic wrap or in an airtight container at room temperature for up to 2 days. If you want it to last longer, store it in the fridge for up to 5 days – just know it might lose a bit of its soft texture when chilled.

Freeze: This cake freezes really well for up to 3 months. Wrap individual slices in plastic wrap, then place them in a freezer bag. That way you can grab just one piece whenever you’re craving something sweet without thawing the whole thing.

Serve: Let frozen slices thaw at room temperature for about an hour, or warm them in the microwave for 20-30 seconds. I actually think this cake tastes even better the next day after all the flavors have had time to meld together!

Estimated Nutrition

Estimated nutrition for the whole recipe (without optional ingredients):

• Calories: 1100-1250
• Protein: 15-18 g
• Fat: 38-45 g
• Carbohydrates: 185-205 g

Ingredients

For the cake:

• 3/4 cup sugar
• 1/3 cup brown sugar
• 1/3 cup butter (I prefer Kerrygold unsalted for this)
• 1 large egg
• 1 tsp vanilla
• 1 1/4 cups flour (I always use King Arthur all-purpose)
• 1/2 cup buttermilk (room temperature, about 70°F)
• 1/2 tsp baking soda
• 1/2 tsp baking powder
• 1 tsp cinnamon
• 1/4 tsp salt
• 1 1/2 cups rhubarb (cut into 1/2-inch slices)
• 2 oz cream cheese, softened

For the topping:

• 1/2 tsp cinnamon
• 1/3 cup brown sugar
• 1/3 cup walnuts (coarsely chopped, about 1/4-inch pieces)

Step 1: Prepare the Mise en Place and Preheat

• 1 1/2 cups rhubarb
• 1/3 cup walnuts

Start by preheating your oven to 350°F.

While it heats, cut the rhubarb into 1/2-inch slices and set aside.

Chop the walnuts into coarse 1/4-inch pieces.

Measure out all your dry ingredients and wet ingredients so everything is ready to go—this prevents overmixing the batter later.

Let your buttermilk sit at room temperature if it isn’t already, as this helps the batter mix more smoothly.

Step 2: Make the Topping Mixture

• 2 oz cream cheese, softened
• 1/3 cup brown sugar
• 1/2 tsp cinnamon
• 1/3 cup walnuts

In a small bowl, combine the softened cream cheese, 1/3 cup brown sugar, and 1/2 tsp cinnamon, stirring until well blended.

Fold in the chopped walnuts gently so they’re evenly distributed throughout the mixture.

Set this aside—I like to make the topping first so it’s ready to go the moment the batter is poured into the pan, ensuring the topping doesn’t sink.

Step 3: Combine the Dry Ingredients

• 1 1/4 cups flour
• 1/2 tsp baking soda
• 1/2 tsp baking powder
• 1 tsp cinnamon
• 1/4 tsp salt

Whisk together the flour, baking soda, baking powder, 1 tsp cinnamon, and salt in a medium bowl.

Make sure these are evenly distributed with no lumps, as this creates a consistent crumb structure in the finished cake.

Step 4: Cream the Butter and Sugars

• 3/4 cup sugar
• 1/3 cup brown sugar
• 1/3 cup butter
• 1 large egg
• 1 tsp vanilla

In a large bowl, beat together the 3/4 cup sugar, 1/3 cup brown sugar, and butter until light and fluffy, about 2-3 minutes.

This incorporates air into the batter, which helps the cake rise evenly.

Add the egg and vanilla extract, beating well until the mixture is pale and smooth, another 1-2 minutes.

Step 5: Build the Batter and Fold in Rhubarb

• dry ingredient mixture from Step 3
• 1/2 cup buttermilk
• butter and sugar mixture from Step 4
• 1 1/2 cups rhubarb slices from Step 1

Alternate adding the dry ingredient mixture from Step 3 and the room-temperature buttermilk to the creamed mixture from Step 4, starting and ending with the dry ingredients—this prevents the batter from becoming tough.

Stir until just smooth, being careful not to overmix.

Gently fold in the rhubarb slices from Step 1, distributing them evenly throughout without crushing them.

Step 6: Assemble and Bake

• rhubarb batter from Step 5
• topping mixture from Step 2

Pour the rhubarb batter into a greased 8×8-inch or 9×13-inch baking dish.

Spread the topping mixture from Step 2 evenly over the batter, making sure it’s distributed to the edges.

Bake for 30-40 minutes in an 8×8 pan or 45 minutes in a 9×13 pan, until a toothpick inserted in the center comes out clean or with just a few moist crumbs.

Step 7: Cool and Serve

Remove the cake from the oven and let it cool in the pan for 10-15 minutes before serving.

This allows the structure to set while the cake is still warm enough to enjoy with the rhubarb and cream cheese topping at their best textures.

I like to serve this cake slightly warm for the most tender crumb and melted topping.

Simple Rhubarb Lunar Cake

Delicious Simple Rhubarb Lunar Cake recipe with step-by-step instructions.

Print Recipe Pin Recipe

Prep Time 20 minutes mins

Cook Time 35 minutes mins

Total Time 55 minutes mins

Servings: 8 slices

Calories: 1175

Ingredients Method

Ingredients

  

For the cake

• 3/4 cup sugar
• 1/3 cup brown sugar
• 1/3 cup butter (I prefer Kerrygold unsalted for this)
• 1 large egg
• 1 tsp vanilla
• 1 1/4 cups flour (I always use King Arthur all-purpose)
• 1/2 cup buttermilk (room temperature, about 70°F)
• 1/2 tsp baking soda
• 1/2 tsp baking powder
• 1 tsp cinnamon
• 1/4 tsp salt
• 1 1/2 cups rhubarb (cut into 1/2-inch slices)
• 2 oz cream cheese, softened

For the topping

• 1/2 tsp cinnamon
• 1/3 cup brown sugar
• 1/3 cup walnuts (coarsely chopped, about 1/4-inch pieces)

Method

 

1. Start by preheating your oven to 350°F. While it heats, cut the rhubarb into 1/2-inch slices and set aside. Chop the walnuts into coarse 1/4-inch pieces. Measure out all your dry ingredients and wet ingredients so everything is ready to go—this prevents overmixing the batter later. Let your buttermilk sit at room temperature if it isn't already, as this helps the batter mix more smoothly.
2. In a small bowl, combine the softened cream cheese, 1/3 cup brown sugar, and 1/2 tsp cinnamon, stirring until well blended. Fold in the chopped walnuts gently so they're evenly distributed throughout the mixture. Set this aside—I like to make the topping first so it's ready to go the moment the batter is poured into the pan, ensuring the topping doesn't sink.
3. Whisk together the flour, baking soda, baking powder, 1 tsp cinnamon, and salt in a medium bowl. Make sure these are evenly distributed with no lumps, as this creates a consistent crumb structure in the finished cake.
4. In a large bowl, beat together the 3/4 cup sugar, 1/3 cup brown sugar, and butter until light and fluffy, about 2-3 minutes. This incorporates air into the batter, which helps the cake rise evenly. Add the egg and vanilla extract, beating well until the mixture is pale and smooth, another 1-2 minutes.
5. Alternate adding the dry ingredient mixture from Step 3 and the room-temperature buttermilk to the creamed mixture from Step 4, starting and ending with the dry ingredients—this prevents the batter from becoming tough. Stir until just smooth, being careful not to overmix. Gently fold in the rhubarb slices from Step 1, distributing them evenly throughout without crushing them.
6. Pour the rhubarb batter into a greased 8x8-inch or 9x13-inch baking dish. Spread the topping mixture from Step 2 evenly over the batter, making sure it's distributed to the edges. Bake for 30-40 minutes in an 8x8 pan or 45 minutes in a 9x13 pan, until a toothpick inserted in the center comes out clean or with just a few moist crumbs.
7. Remove the cake from the oven and let it cool in the pan for 10-15 minutes before serving. This allows the structure to set while the cake is still warm enough to enjoy with the rhubarb and cream cheese topping at their best textures. I like to serve this cake slightly warm for the most tender crumb and melted topping.