01 - Preheat your oven to 350°F and 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 another bowl, combine the mashed bananas, melted butter, eggs, vanilla extract, and milk. Whisk until smooth and fully incorporated.
04 - Pour the wet ingredients into the dry ingredients. Stir gently with a spatula until just combined; do not overmix to ensure tender muffins.
05 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full.
06 - Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
07 - Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before serving.