Preheat your oven to 350°F and grease a standard 12-cup muffin pan with butter or nonstick spray. Mash the bananas in a small bowl until completely smooth—no lumps should remain, as this ensures even distribution throughout the batter. Dice the strawberries into small 1/4-inch pieces and set aside on a paper towel to absorb any excess moisture, which prevents the muffins from becoming soggy. Having everything prepped and ready before you start mixing ensures the batter comes together quickly and bakes at its best.

In a medium mixing bowl, combine the softened butter and sugar, then beat with an electric mixer on medium speed for about 2-3 minutes until the mixture is light, fluffy, and pale in color. This process incorporates air into the batter, which helps the muffins rise properly and achieve a tender crumb. I like using Kerrygold butter for its rich, golden flavor that really shines through in the final muffins.

Add the room-temperature egg, mashed bananas from Step 1, and vanilla extract to the creamed butter and sugar mixture. Beat on medium speed for about 1 minute until everything is well combined and the batter looks smooth and cohesive. The room-temperature egg incorporates more evenly into the batter, creating a more uniform texture throughout the muffins.

In a separate small bowl, whisk together the flour, baking powder, baking soda, and salt. Add this dry mixture to the wet ingredients from Step 3 and stir gently with a wooden spoon or spatula just until combined—do not overmix, as this can develop the gluten and result in dense, tough muffins. A few small lumps are perfectly fine; they'll disappear during baking.

Gently fold the diced strawberries from Step 1 and the optional raisins into the batter using a rubber spatula, being careful not to overmix or crush the fruit pieces. Folding rather than stirring keeps the batter tender and ensures the fruit stays distributed throughout rather than sinking to the bottom. I find folding by hand gives you much better control over texture than using an electric mixer at this stage.

Divide the batter evenly among the greased muffin cups, filling each about two-thirds to three-quarters full. Bake in the preheated 350°F oven for 18-20 minutes, until a toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs clinging to it. The muffins should be golden brown on top and spring back slightly when gently touched.

Remove the muffin pan from the oven and let the muffins cool in the pan for 5 minutes—this allows them to set enough to handle without falling apart. Turn the muffins out onto a wire cooling rack to cool completely. They're delicious served warm with a pat of butter or enjoyed at room temperature.