Preheat oven to 350°F and line a muffin tin with paper liners.
Mash bananas in a large mixing bowl until smooth.
Whisk in melted butter, sugar, egg, and vanilla until combined.
Sprinkle baking soda, baking powder, and salt over the mixture, then stir in.
Add flour and fold gently until just combined (don’t overmix).
Spoon batter evenly into muffin cups, filling about 3/4 full.
Bake for 18–22 minutes, or until a toothpick comes out clean.
Cool slightly on a wire rack before enjoying warm.