Preheat oven to 425°F. Grease the muffin pan with cooking spray
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. Whisk in the melted butter, brown sugar, egg, vanilla extract, and milk.
Pour the dry ingredients into the wet ingredients, then beat or whisk until combined. Fold in the nuts and chocolate chips.
Spoon the batter into the muffin pan, filling them to the top. Bake for 5 minutes. Then, keeping the muffins in the oven, reduce the oven temperature to 350°F. Bake for an additional 15-18 minutes or until a toothpick inserted in the center comes out clean.
The total time these muffins take in the oven is about 20-23 minutes, give or take.
Allow the muffins to cool for 10 minutes in the muffin pan, then transfer to a wire rack to continue cooling.