Preheat oven to 425°F (218°C). Line or grease a 12-count muffin pan.
In a medium bowl, whisk flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In a large bowl, mash bananas until smooth.
Add melted butter, brown sugar, egg, vanilla extract, and milk. Mix well.
Fold dry ingredients into wet ingredients until just combined.
Stir in nuts or chocolate chips if desired.
Divide batter evenly into muffin cups, filling each to the top.
Bake the banana muffins for 5 minutes at 425°F.
Without opening the oven, reduce temperature to 350°F (177°C) and bake another 16–18 minutes.
Cool in pan for 5 minutes, then transfer to a wire rack.