Preheat your oven to 425°F (218°C). Prep your 12-count pan with pretty cupcake liners or nonstick spray.
Whisk the flour, baking soda, baking powder, cinnamon, salt, and nutmeg in a bowl. Set that bowl aside for a second.
Mash bananas in a bigger bowl (or your stand mixer bowl). At medium speed, mix in the unsalted butter, egg, brown sugar, milk, and vanilla extract until creamy.
Pour dry ingredients (from Step 2) into wet ingredients, then mix well. Toss in chocolate chips or nuts (optional). Your batter will become thick.
Scoop the muffin batter into your liners and fill them to about ¾. Pop them in the oven for about 5 minutes, then dial the temperature to 350°F and let them bake for another 16 to 18 minutes.
Stick a toothpick in the middle — when it comes out mostly clean, they're good to go (21-23 minutes in total, or 12 to 15 mins at 350°F if you make mini muffins)
Let them sit in the pan for 5-7 minutes, then transfer them to a rack to chill completely. Enjoy!