Preheat & Prepare – Preheat the oven to 425°F (218°C). Line a 12-cup muffin pan with liners or lightly grease with nonstick spray.
Mix Dry Ingredients – In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
Combine Wet Ingredients – In a large bowl, mash the bananas. Stir in the melted butter, brown sugar, egg, vanilla extract, and milk. Mix until smooth.
Combine & Fold – Gently add the dry ingredients to the wet mixture, stirring until just combined. If using nuts or chocolate chips, fold them in now. The batter will be thick.
Fill & Bake – Divide the batter evenly among muffin cups, filling each to the top. Bake at 425°F (218°C) for 5 minutes, then reduce the temperature to 350°F (177°C) and bake for 16-18 more minutes, or until a toothpick inserted in the center comes out clean.
Cool & Serve – Let muffins cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.