Preheat oven to 425°F (218°C). Line a 12-count muffin pan with paper liners or grease with nonstick spray.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Set aside.
In a large mixing bowl, mash the bananas until smooth.
Add melted butter, brown sugar, egg, vanilla extract, and milk to the bananas. Whisk until combined.
Gradually fold the dry ingredients into the wet ingredients, stirring gently until just combined. Do not overmix.
If using mix-ins, gently fold them into the batter.
Divide the batter evenly among the muffin cups, filling each to the top.
Bake for 5 minutes at 425°F (218°C), then reduce oven temperature to 350°F (177°C) and bake for an additional 16-18 minutes, or until a toothpick inserted in the center comes out clean.
Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Enjoy warm or store in an airtight container for up to 5 days. Muffins can also be frozen for up to 3 months.