Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners or grease the cups with butter or cooking spray.
Mash the bananas in a large bowl until smooth with some small lumps. Add eggs, melted butter (or oil), and milk to the mashed bananas, and whisk until combined.
Combine the dry ingredients in another bowl: whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg (if using).
Add the dry ingredients to the wet ingredients. Gently fold them together using a spatula until just combined. Be careful not to overmix; a few lumps are fine.
Fold in optional add-ins like chocolate chips, walnuts, or dried fruit.
Spoon the batter into the muffin tin, filling each cup about 2/3 full.
Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the tin for 5 minutes before transferring the muffins to a wire rack to cool completely.