Preheat your oven to 425°F (218°C). Line a 12-count muffin pan with cupcake liners or spray with nonstick spray.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In a large bowl, mash the ripe bananas until mostly smooth. Add melted butter, brown sugar, egg, vanilla extract, and milk. Whisk until fully combined.
Add the dry ingredients into the wet ingredients and mix gently until just combined. Do not overmix. Fold in nuts or chocolate chips if using.
Divide the batter evenly into the muffin cups, filling each to the top.
Bake the Quick & Easy Banana Muffins for 5 minutes at 425°F (218°C). Without opening the oven, reduce the temperature to 350°F (177°C) and continue baking for 16–18 minutes.
Check doneness with a toothpick inserted into the center. It should come out clean.
Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack.