Combine the rhubarb pieces, raspberries, sugar, lemon juice, vanilla, and pinch of salt in a saucepan over medium heat. Stir occasionally until the rhubarb breaks down and the mixture becomes jam-like, about 15-20 minutes. The fruit should be very soft and syrupy. Remove from heat and let cool completely—this will be used both as a cake layer and folded into the buttercream. I find that straining out the raspberry seeds at this point (as with the jam) creates a smoother, more elegant filling, though it's optional.

Preheat your oven to 350°F. In a medium bowl, whisk together the flour, baking powder, salt, and cinnamon—this ensures even distribution of the leavening agent and spice. In a large bowl, beat the room-temperature butter and canola oil together until combined, then add the sugar and beat until light and fluffy, about 2-3 minutes. Add eggs one at a time, beating well after each addition to incorporate air into the batter. Alternate adding the dry ingredient mixture and sour cream (mixed with 1 1/2 tbsp vanilla and 3/4 cup milk) in three additions each, starting and ending with dry ingredients. Mix on low speed just until combined to avoid overworking the batter.

Divide the batter evenly between two greased 6-inch round cake pans (or use one pan and bake in two batches). Bake at 350°F for 25-30 minutes, until a toothpick inserted in the center comes out clean and the tops spring back when lightly touched. Remove from the oven and let cool in the pans for 10 minutes, then turn out onto wire racks to cool completely. This cooling step is crucial—a warm cake will melt the buttercream and make layering difficult.

Create a double boiler by placing a heatproof bowl over a pot of gently simmering water (the bowl shouldn't touch the water). Add the egg whites and 1 cup sugar to the bowl, whisking constantly for 5-10 minutes until the mixture reaches 160°F on an instant-read thermometer and feels completely smooth with no grittiness—this pasteurizes the eggs and dissolves the sugar. Transfer to an electric mixer and beat on high speed for 7-10 minutes until thick, glossy, and cooled to room temperature. The mixture should hold stiff peaks.

With the mixer on medium-low speed, add the softened butter 2 tablespoons at a time, allowing each addition to incorporate fully before adding the next (this may take 5-7 minutes total). Once all butter is incorporated, add the vanilla and beat briefly to combine. Gently fold in the cooled rhubarb-raspberry compote from Step 1 and the strained raspberry jam, using a spatula to fold just until marbled throughout—don't overmix or you'll lose the pretty swirl. I like to leave some jam visible rather than fully incorporate it, which keeps pockets of intense fruit flavor and a beautiful presentation.

Place the first cooled cake layer on a cake board or serving plate and spread a thin layer of the fruit-swirled buttercream from Step 5 on top. Add another thin layer of buttercream, then add the second cake layer and press gently. Chill the assembled cake for 30 minutes in the refrigerator—this firms up the filling and makes frosting the exterior cleaner and easier. Remove from the fridge and frost the top and sides with the remaining buttercream, using an offset spatula for a smooth finish or creating decorative swirls as desired. Chill for at least 30 minutes before serving so the frosting sets properly.