Preheat oven to 350 degrees. Coat an 8 or 12-cup muffin tin with cooking spray. In a small bowl, melt the butter. Add oats, brown sugar and cinnamon and stir to combine.
Set aside. In the bowl of a stand mixer, cream bananas, brown sugar, eggs and oil until fluffy. In a separate medium bowl, whisk together oat flour, oats, baking powder, baking soda and salt.
While the mixer is running, slowly dump dry ingredients into the banana mixture, just until combined. Fold in chocolate chips, if using.
Spoon batter into muffin tin, about ⅔ of the way full. Top each muffin with a spoonful of topping. Bake 18-22 minutes, until toothpick inserted into the center of a muffin comes out clean. Allow to cool.