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, sugar, baking powder, baking soda, and salt.
In another bowl, mix the mashed bananas, egg, melted butter, and vanilla extract.
Combine the wet ingredients with the dry ingredients and stir until just combined.
Spoon the batter into the prepared muffin tin, filling each cup about ¾ full.
Bake for 15 minutes or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for a few minutes, then transfer to a wire rack to cool completely.