Preheat oven to 400 degrees. Grease bottoms only of a 12 cup muffin tin, or line with paper baking cups and set aside.
In a large bowl, beat together milk, oil, and egg. Add banana and mix well. Stir in flour, sugar, baking powder, and salt, just until mixed. Batter will be lumpy. Divide evenly among muffin cups.
To make crumb topping, cut butter into flour and sugar until crumbly. Top batter with crumb topping.
Bake 20 to 25 minutes, until golden brown. If baked in a greased pan, let cool 5 minutes in pan before removing to wire racks.