Preheat the oven to 425°F. Line a standard 12-cup muffin tin with paper liners and set aside.

In a medium bowl, whisk together flour, baking powder, baking soda, and kosher salt.

In a small bowl, mash the bananas with a fork until mostly smooth.

In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer), cream together the softened butter and granulated sugar on medium-high speed until light and fluffy, about 2 minutes. Scrape down the sides of the bowl as needed.

Add the egg and mix until fully combined.

Add the mashed bananas, buttermilk, and vanilla extract. Mix on low speed until just combined.

Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Do not overmix.

In a small bowl, toss the diced strawberries with the tablespoon of flour.

Remove the bowl from the mixer. Gently fold the floured strawberries into the batter using a rubber spatula.

Scoop the batter evenly into the prepared muffin cups, filling each about ¾ full.

Let the muffins cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.