Preheat your oven to 350°F (175°C). Grease a mini muffin tin or line with paper liners.
In a mixing bowl, whisk together the flour, baking powder, baking soda, and salt.
In another bowl, mix the mashed bananas, sugar, melted butter, egg, and vanilla extract until well combined.
Add the dry ingredients to the wet ingredients and mix until just combined. Do not overmix.
Spoon the batter into the prepared muffin tin, filling each cup about ¾ full.
Bake for 12-15 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the tin for a few minutes, then transfer to a wire rack to cool completely.