In a large bowl, whisk together the mashed bananas, melted butter, granulated sugar, dark brown sugar, egg, and vanilla until well combined then set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt. Pour the dry ingredients into the wet ingredients and whisk until just combined.
Cover the bowl with plastic wrap and allow the muffin batter to rest for 15 minutes. While the batter is resting, preheat the oven to 425°F (220°C) and line a 12-cup muffin pan with cupcake liners, skipping every other cup.
Spoon the batter into the prepared muffin pan, filling each cup to the top; you’ll use about 4½-5 tablespoons of batter per cup. Sprinkle the tops with turbinado sugar, if using.
Bake the muffins for 5 minutes. Keeping the muffins in the oven, reduce the oven temperature to 350°F (180°C), and bake them for 15 to 17 minutes, or until a toothpick inserted into the center comes out clean. Allow the muffins to cool for 10 minutes in the pan before removing them to a wire rack to cool completely. Repeat with any remaining batter.