Preheat oven to 375º F and coat a large loaf pan or mini loaf pans with cooking spray. Using an electric mixer on medium speed, beat the banana, egg, sugar, vanilla and applesauce. Sift the flour, salt, baking powder and baking soda in another bowl, stir until combined. Beating on low speed, add the dry ingredients to the banana mixture, beat until moist. In a mixing bowl combine the brownie topping ingredients and mix until crumbly. Spread the batter into the loaf pan(s) adding a layer of brownie topping in the middle and again on top. Bake for 50-60 minutes for a large loaf, 25-28 for mini loaves; until a tester comes out mostly clean. Cool in the pan for 10 minutes before turning onto a wire rack, cool completely.
The topping recipe is for a full batch, you won't need quite that much, just toss the leftovers into a freezer bag for later.
Recipe by It Bakes Me Happy at http://itbakesmehappy.com/2013/06/brownie-crumble-banana-bread.html