01 - Preheat oven to 350°F and line a 12-cup muffin tin with paper liners or grease lightly 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 mixture into dry ingredients. Fold gently until just combined—do not overmix or muffins will become dense.
05 - Divide batter evenly among the 12 prepared muffin cups, filling each about three-quarters full.
06 - Bake for 18-22 minutes until a toothpick inserted into the center comes out clean.
07 - Let muffins rest in the tin for 5 minutes, then transfer to a wire rack to cool completely.