Preheat oven to 350 degrees and butter an 8x4 loaf pan.
Mix butter and bananas in a large mixing bowl. Add in egg, sugar and vanilla then mix until well incorporated. Sprinkle baking soda and salt on top then mix in.Carefully mix flour into wet mixture being sure not to over mix. Then pour mixture into loaf pan.
For the topping combine dry ingredients and cut butter into mixture until it becomes sand-like and crumbly. Sprinkle this topping generously on top of the bread then bake for 50-60 minutes.
This is best served warm with a good, strong cup of coffee.