Adjust an oven rack to the middle position and preheat oven to 375 degrees. Coat a muffin pan with nonstick spray or line with paper liners. In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
In a medium bowl, whisk together banana, melted butter, sugar, and egg. Using a rubber spatula, carefully stir in flour mixture. Muffin batter will be lumpy, do not overmix. Fold in nuts, if using.
Divide batter among prepared muffin cups (I like using a measuring scoop). Bake until a toothpick inserted in the middle comes out clean with a few crumbs attached, about 20 to 25 minutes. Cool 5 minutes in pan before transferring to a wire rack to cool completely.
Notes
Bananas: Use fully-ripe or slightly overripe bananas for the best flavor (brown spots mean naturally sweet!). If your bananas aren't quite ripe yet, you can ripen the bananas in a 300-degree oven: Arrange unpeeled bananas on a baking sheet, then bake until banana skins are completely black all over, about 15 to 20 minutes. Cool completely, scoop banana pulp from peel, and mash.
Yield: My Banana Muffin recipe creates 12 regular-sized muffins.
Storage: Store leftovers covered at room temperature for up to 4 days.