Preheat oven to 350℉. Lightly grease 12 muffins cups, or line with muffin liners.
Mix together all-purpose flour(1 ½ cups), baking soda (1 teaspoon), baking powder(1 teaspoon), cinnamon(½ teaspoon), and nutmeg(⅛ teaspoon). In another bowl, mix together mashed bananas(3 ripe), granulated sugar(½ cup), light brown sugar(¼ cup), egg(1 large), and melted butter(¼ cup).
Add banana mixture into the flour mixture and stir until just moistened. Scoop 3 tablespoons of batter in each muffin cup (about ¾ full) and lightly tap pan on counter to even out the tops.
Bake at 350℉ for 16-20 minutes or until toothpick comes out clean.