Preheat oven to 375 degrees F. Lightly grease 12 muffin cups, or line with muffin liners.
For the topping : In a medium bowl, mix flour, sugar and cinnamon. Add butter and mix with a fork or pastry cutter until crumbly.
For the muffins : In large bowl, combine dry ingredients(flour, baking soda, baking powder and salt). Set aside.
In another bowl, combine mashed bananas, sugar, slightly beaten egg, vanilla and melted butter. Mix well. Stir into dry ingredients just until moistened. Fill muffin cups two-thirds full.
Sprinkle topping over muffin batter. Bake at 375 degrees for 18 to 20 minutes or until a toothpick inserted into center of a muffin comes out clean.
Cool in pan 10 minutes before removing to a wire rack.