Preheat the oven to 350°F. Line a muffin tin with 12 muffin liners. Mix together the egg, bananas, milk, melted butter and sugar using a mixer until blended. Add in the flour and baking powder until blended, being careful not to over mix. If using chocolate chips stir them in by hand.
Fill the muffin tin with batter, filling each about 3/4 full. Mix together all the ingredients for the oat crumble topping. Sprinkle 1 tablespoon of the topping over each muffin and bake for 30 minutes until the muffins are golden on top and a toothpick inserted into the center comes out clean and not gooey.