Preheat your oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners or spray with non-stick cooking spray.
In a large bowl, mash the ripe bananas until mostly smooth with a few small chunks remaining.
Add the melted butter, granulated sugar, and brown sugar to the mashed bananas. Whisk until well combined.
Beat in the eggs one at a time, then stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking soda, salt, cinnamon, and nutmeg.
Fold the dry ingredients into the wet mixture using a rubber spatula, stirring just until combined. Do not overmix.
If using, gently fold in the nuts or chocolate chips.
Divide the batter evenly among the prepared muffin cups, filling each about ¾ full.
Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean with a few moist crumbs.
Allow the muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.