Preheat the oven to 375°F (190°C) and line a muffin tin with paper liners.
In a bowl, mash the bananas until smooth.
Add the egg, oatmeal, baking powder, baking soda, brown sugar, ground cinnamon, flour, granulated sugar, salt, topping, vanilla extract, vegetable oil, butter, and milk. Mix well until combined.
Divide the batter evenly into the muffin tin cups.
Bake for about 20-25 minutes or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool before serving.
