Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease.

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

In another bowl, mash the bananas. Stir in sugar, melted butter, egg, and vanilla until well combined.

Add the wet ingredients to the dry ingredients and mix gently—just until combined (a few lumps are okay!).

Fold in the blueberries with a spatula.

Divide the batter evenly among the muffin cups.

Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.

Let cool in the pan for 5 minutes, then transfer to a wire rack.