Preheat oven to 350°F (175°C) and line a 12-cup muffin tin with liners.
In a large bowl, mash the bananas until smooth.
Add melted butter, sugar, egg, milk, and vanilla. Mix until combined.
Stir in oats and let sit for 5 minutes.
Add flour, baking powder, baking soda, cinnamon, and salt. Stir gently until just combined.
Divide the batter evenly into the prepared muffin cups.
Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean.
Cool for 5 minutes in the pan, then transfer to a wire rack to cool completely.