Preheat your oven to 350°F (175°C). Grease a muffin tin or line with muffin liners.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt.
In another bowl, mix the mashed bananas, sugar, egg, and melted butter.
Stir the banana mixture into the flour mixture until just combined. Do not overmix.
Fold in the chopped nuts.
Spoon the batter into the prepared muffin tin, filling each cup about ¾ full.
Bake for 20 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.