Preheat the oven to 425°F. Grease every other cup of a 12-cup muffin tin or line with paper liners.
Melt the butter until about 90% melted in a large microwave-safe mixing bowl. Whisk until completely melted.
1/4 cup salted butter
Add banana and mash until well-combined.
1/2 cup mashed ripe banana
Whisk in the brown sugar, egg, and vanilla extract until smooth.
1/3 cup light brown sugar, 1 large egg, 1/2 teaspoon vanilla extract
Add the flour, baking powder, baking soda, ground cinnamon, and salt. Stir until just incorporated; do not overmix.
1 cup purpose flour, 1/2 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon ground cinnamon, 1/4 teaspoon salt
Divide the batter between the prepared muffin tin cups.
Sprinkle the tops of the batter with sugar.
Bake for 5 minutes.
Without opening the oven, reduce the baking temperature to 350°F, and continue baking for about 10-15 minutes, or until a wooden toothpick inserted in the center of the muffin comes out with just a couple of crumbs.
Let the muffins cool in the pan for 5-10 minutes before removing to a wire rack to finish cooling.
1 tablespoon turbinado or granulated sugar
Notes
Salted butter: If using unsalted butter, add an additional pinch or two of salt.
Ripe banana: A ripe banana will have brown spots of be darker yellow. It's important to use ripe or overripe bananas for the best flavor.