Preheat oven to 350°F (175°C)
Line a 12-cup muffin tin with paper liners or lightly grease
In a bowl, mash 3 ripe bananas (about 1 1/2 cups)
Whisk in 1/3 cup melted butter (or neutral oil) and 1/2 cup brown sugar (or 1/3 cup sugar)
Whisk in 1 large egg and 1 tsp vanilla extract
In a separate bowl, whisk together 1 1/2 cups all-purpose flour, 1 tsp baking soda, 1/2 tsp baking powder, and 1/2 tsp salt
Add dry ingredients to wet ingredients and mix just until combined
Fold in 1/2 cup chopped walnuts or chocolate chips (optional)
Divide batter evenly among muffin cups (fill about 3/4 full)
Bake 16–20 minutes, until a toothpick comes out clean or with a few moist crumbs
Cool in pan 5 minutes, then transfer muffins to a rack to cool completely