Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius). Prepare the muffin tin with spray (this will give the muffins crisp outsides) or line it with 12 paper liners.
In the bowl of a stand mixer, beat together the sugar and butter. Add the egg and vanilla, beating just until incorporated.
Mash the bananas on a plate using a fork. Add the bananas to the butter-sugar mixture. Beat until smooth.
In a medium-sized mixing bowl, whisk together the flour, baking soda, and salt. With mixing speed on low, alternate the dry ingredients with the buttermilk. Mix until combined. Remove the bowl from the stand and fold in the diced strawberry bits.
Pour the muffin batter into the prepared muffin tin, filling each ⅔ of the way full. Lay additional strawberry bits on top, if desired.
Place the muffin tin in the oven and bake for approximately 23–26 minutes, or until a cake tester or toothpick inserted into the center comes out clean and the muffin tops are golden brown. Remove from the oven and let cool for 10 minutes, then transfer the muffins onto a wire rack to cool completely.
Notes
STORE leftover Banana Strawberry Muffins in an airtight container at room temperature for up to 2 days. To extend freshness, refrigerate them for up to 5 days.If you want to keep them longer, FREEZE the muffins once they’ve completely cooled. Wrap individually in plastic wrap and place in a freezer-safe bag or container—they’ll keep for up to 2 months. Thaw them in the refrigerator or on the counter for a few hours. To REHEAT, warm a muffin in the microwave for about 20–30 seconds or let it come to room temperature on the counter.