Preheat the oven to 425°F. Line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.
In a medium bowl, whisk together the banana, butter, both sugars, eggs, and vanilla until well incorporated but the bananas are still a bit lumpy. Pour the banana mixture into the flour mixture and stir until combined. Divide the batter among each paper liner (they will be almost full).
Bake for 5 minutes, then reduce the oven temperature to 350°F and continue baking for another 15 minutes or until the centers are springy to the touch and the tops are golden brown. Let the muffins cool in the pan for a few minutes, then remove and place on a wire rack. Serve warm or cool completely on the rack.
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Notes
Measure the flour correctly. Using too much flour is a surefire way to end up with dry muffins. The best way to measure flour is using a kitchen scale. If you don’t have one, fluff up the flour in its container, spoon it into your measuring cup, and level off the top with a butter knife.
Use room temperature ingredients. Set out the eggs about 30 minutes before you start baking, so they warm to room temperature. This will help them incorporate better into the batter.
Ripen bananas in 10 minutes: If your bananas aren't ripe, place them on a baking sheet and bake at 350°F for about 10 minutes or until blackened. Let cool completely, then peel and mash.
Don’t overmix the batter. Stop mixing when everything is just combined. Mixing too much yields dense, tough muffins.
Mix-in options: You can add chopped nuts like pecans or walnuts, as well as chocolate chips (or just try my banana chocolate chip muffins). Add about ½ cup of nuts or 1 cup of chocolate chips to the batter right at the end, when the batter is just about ready.