Preheat oven to 350 °F. Spray a loaf pan or butter and lightly flour it.
In a separate bowl from the crumb topping mixture, combine flour, salt, baking soda and cinnamon. Set aside.
Beat the butter, banana and granulated sugar on high speed until smooth and creamy, add egg and vanilla and continue to beat until mixed. Be sure to scrape down the sides of the bowl.
Add the dry ingredients to the batter and mix just until blended. Over blending will result in a heavier cake.
Pour half of mixture in loaf pan and distribute evenly to the edges. Place half of the crumb topping on.
Pour remaining batter gently on the top of the streusel, moving the batter to edges with a knife, being careful not to pull the layer of streusel in with it.
Pour remaining streusel on the top and distribute evenly.
Bake in oven for 60-70 minutes.
Tip: I usually loosely cover the loaf pan after 45 minutes so the topping doesn't brown too much.