Prepare the oven: Preheat your oven to 425°F (220°C). Line a 12-cup muffin tin with paper liners.
Mix the dry ingredients: In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.
Mix the wet ingredients: In a separate medium bowl, whisk together the mashed bananas, melted butter, granulated sugar, brown sugar, eggs, and vanilla. Mix until well combined, leaving a few small banana lumps for extra texture.
Combine: Pour the banana mixture into the dry ingredients. Gently stir until just combined. Avoid overmixing, as this can make the muffins dense.
Fill the muffin tin: Divide the batter evenly among the 12 paper liners. The cups should be almost full.
Bake: Bake at 425°F for 5 minutes. Without opening the oven, reduce the temperature to 350°F (175°C) and bake for another 15 minutes, or until the tops are golden brown and the centers spring back when gently touched.
Cool: Let the muffins cool in the pan for a few minutes, then transfer them to a wire rack.
