Preheat oven to 350 degrees F (175 degrees C). Coat muffin pans with non-stick spray, or use paper liners. Sift together the flour, baking powder, baking soda, and salt; set aside.
Combine bananas, sugar, egg, and melted butter in a large bowl. Fold in flour mixture, and mix until smooth. Scoop into muffin pans. You may add chopped walnuts to the top of the muffins for some extra crunch.
Bake in preheated oven. Bake mini muffins for 10 to 15 minutes, and large muffins for 25 to 30 minutes. Muffins will spring back when lightly tapped.