Preheat oven to 425 °F. Line muffin pan with cupcake liners.
In the bowl of your stand mixer, add bananas, butter, brown sugar, egg, vanilla, and milk. Beat on medium speed until well incorporated.
Meanwhile, whisk flour, baking powder, baking soda, salt, cinnamon, and nutmeg together. Add dry ingredients into the bowl of your mixer, and beat until combined. If adding nuts or chocolate chips, fold them in by hand.
Fill cupcake liners all the way to the top with batter. This recipe will yield exactly 12 muffins. Bake for 5 minutes at 425 °F, then while keeping the muffins in the oven with the door closed, lower oven temperature to 350 °F and bake for an additional 16-18 minutes. Allow muffins to cool for 5 minutes in the pan, then transfer to a wire cooling rack.