Preheat the oven to 425°F and prepare a muffin pan with paper liners or non-stick spray. Melt the butter in a microwave-safe bowl and set aside.
Place the flour, baking soda, baking powder, cinnamon, nutmeg, and salt in a medium mixing bowl and mix together using a whisk or fork and set aside.
Peel and place the ripe bananas In a large mixing bowl (or in the bowl of your stand mixer or mix by hand) and beat on low until mashed and smooth.
Beat in the brown sugar, egg, vanilla extract, milk, and melted butter on medium speed until everything has combined. Pour the dry ingredients into the wet ingredients, then beat or stir until just combined. Give the mixture a turn or two with your spatula/spoon and mix in the nuts by hand if you are adding them.
Place the batter in the muffin pan evenly across 12 sections. The cups should be almost full. Bake for 5 minutes at 425°F and then reduce the temperature to 350°F. Don't open the door, just reduce the heat. This step will help the muffins rise in the pan and bake evenly. Continue baking for 15-18 more minutes until a toothpick comes out clean. Remove the muffins from the oven and let them cool in the pan for 5 minutes before placing them on a cooling rack to cool completely.