Position oven rack to the middle of the oven and preheat to 350°F (175°C). Prepare non-stick 12-cup muffin pan and/or use muffin liners.
In a large bowl, add mashed banana, melted butter, brown sugar and whisk together. Add egg, vanilla and milk and whisk again until combined.
Add flour, baking powder, baking soda, salt, cinnamon and nutmeg and continue to whisk until ingredients are well-distributed and no flour bits remain. If adding walnuts or chocolate chips, fold them in now. The batter will be thick.
Spoon batter into muffin liners, filling them all the way to the top. Bake for 20-22 minutes. Allow the muffins to cool for 5 minutes in the pan, then transfer to a wire cooling rack.