Start by preheating your oven to 375°F (190°C). Grease your 12-cup muffin tin with cooking spray or use paper liners so the muffins release easily.
Peel your ripe bananas and mash them in a large bowl until fairly smooth. Stir in the beaten egg, melted and cooled butter, and vanilla extract until well combined.
In another bowl, whisk together the flour, brown sugar, baking soda, and salt. Pour the dry mix into the wet banana mixture and stir gently until just combined—don’t overmix or your muffins will be tough. If using chocolate chips or walnuts, fold them in now.
Spoon batter into the muffin tin, filling each cup about three-quarters full. Bake for roughly 20 minutes, or until a toothpick inserted into the center of a muffin comes out with moist crumbs. Let the muffins cool briefly in the pan, then transfer them to a wire rack to finish cooling.