Preheat your oven to 425°F (220°C). Line a 12-cup muffin tray with paper liners and set aside.
Mash the bananas using a fork or potato masher. Set aside.
In a stand mixer or using a hand-held mixer, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.
Add the egg and vanilla extract and mix again until fully combined.
Blend in the mashed bananas and milk. (The mixture may look a little separated, that’s okay!)
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Gently add the dry ingredients to the wet ingredients. Use a rubber spatula to mix just until the flour disappears. Be careful not to overmix if you want soft, fluffy muffins!
If you’re adding chocolate chips, walnuts, or other mix-ins, fold them in now.
For the most perfect muffin tops, let the batter rest for 10–15 minutes at room temperature (a quick rest period helps the muffins rise higher in the oven).
Divide the batter evenly between the liners, filling each one all the way to the top. This recipe makes 12 muffins. Don’t worry if they look full!
Bake at 425°F for 5 minutes, then (without opening the oven door) lower the temperature to 375°F and continue baking for 13–15 minutes, or until a toothpick inserted in the center comes out clean.
Let the muffins cool in the tray for about 5 minutes, then transfer to a wire rack to cool completely. Or enjoy warm with a hot cup of coffee!