Preheat the oven to 425°F (218°C).
Line a 12-cup muffin pan with paper liners or lightly grease with nonstick spray.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In a large mixing bowl, mash the bananas until mostly smooth.
Add melted butter, brown sugar, egg, vanilla extract, and milk. Whisk until well combined.
Add the dry ingredients to the wet ingredients and stir until just combined.
Fold in walnuts, pecans, or chocolate chips if using.
Divide the batter evenly among the muffin cups, filling each to the top.
Bake at 425°F for 5 minutes.
Without opening the oven door, reduce the temperature to 350°F (177°C).
Continue baking for 16–18 minutes, or until a toothpick inserted into the center comes out clean.
Allow the Quick & Easy Banana Muffins to cool in the pan for 5 minutes before transferring to a wire rack.