Preheat oven to 350 degrees F. Prepare a muffin tin with spray (this will give the muffins nice crusty outsides) or 12 paper liners.
In the bowl of a stand mixer, beat together sugar and butter. Add egg and vanilla, beating just until incorporated.
Mash bananas on a plate using a fork. Add bananas to the butter-sugar mixture. Beat until smooth.
In a medium size mixing bow, whisk together flour, baking soda, and salt. With mixing speed on low, alternate dry ingredients with buttermilk. Mix until combined. Remove bowl from stand and fold in diced strawberry bits.
Pour batter into prepared muffin tin, filling each ⅔ of the way full, lay strawberry bits on top, if desired.
Place in oven and bake for approximately 23-26 minutes, or until a cake tester inserted into the center comes out clean and the muffins tops are golden brown. Remove from oven and let cool for 10 minutes, then transfer muffins onto wire rack to cool completely.