Grease a standard muffin pan (or line with paper muffin liners). Preheat oven to 350 degrees F.
Mash bananas in a mixing bowl. Add egg, vanilla, brown sugar, granulated sugar, and cinnamon and stir well to combine.
In a separate small bowl stir together flour, baking powder, baking soda and salt. Add to banana mixture and stir, just to combine.
Gently stir in melted butter. Don't over-mix the batter.
Spoon the batter into prepared muffin pan and bake for 18-25 minutes. Makes 12 muffins.
