01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or lightly grease with butter.
02 - In a large bowl, whisk together flour, granulated sugar, brown sugar, baking powder, baking soda, salt, and cinnamon until well blended.
03 - In a separate bowl, combine mashed bananas, melted butter, eggs, vanilla extract, and milk. Whisk until smooth and fully incorporated.
04 - Pour wet ingredients into dry ingredients. Fold gently with a spatula until just combined—do not overmix as this will result in dense muffins.
05 - Divide batter evenly among the 12 prepared muffin cups, filling each about three-quarters full.
06 - Bake for 18 to 22 minutes, or until a toothpick inserted into the center comes out clean and tops are golden brown.
07 - Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely. Serve warm or at room temperature.