In a separate bowl, cream together butter and brown sugar. Stir in eggs and mashed bananas until well blended.
Stir banana mixture into flour mixture; stir just to moisten. Pour batter into a greased loaf pan.
Mix streusel ingredients together and sprinkle on top of batter.
Bake at 350° for 55-60 minutes, until a toothpick inserted into center of the loaf comes out clean.
Let bread cool in pan for 10 minutes, then turn out onto a wire rack.