Preheat oven to 425°F (218°C). Spray a 12-count muffin pan with nonstick spray or use cupcake liners.

Whisk the flour, baking powder, baking soda, salt, cinnamon, and nutmeg together in a medium bowl. Set aside.

In a large bowl, mash the bananas. On medium speed, beat in the melted butter, brown sugar, egg, vanilla extract, and milk.

Pour the dry ingredients into the wet ingredients, then mix until combined. If adding nuts or chocolate chips, fold them in now. Batter will be thick.

Spoon the batter into liners, filling them all the way to the top.

Bake for 5 minutes at 425°F, then reduce the oven temperature to 350°F (177°C) without removing the muffins. Bake for an additional 16–18 minutes or until a toothpick inserted in the center comes out clean.

Allow the muffins to cool for 5 minutes in the muffin pan, then transfer to a wire cooling rack to continue cooling.