Combine de egg, melted butter, milk and mashed bananas.
In another bowl combine the flour, sugar, baking powder, salt and cinnamon.
Combine both mixtures and mix until they are incorporated.
Divide batter among muffin cups.
Oatmeal Crumb Toppping
Mix ingredients and top muffins with it.
Bake until a toothpick inserted in the center of a muffin comes out clean, 15 to 20 minutes, rotating pan halfway through. Let cool in pan 10 minutes; transfer muffins to a rack to cool a little bit more.