Preheat oven to 35Line a square baking pan with parchment paper or spray with non stick baking spray.
In a small mixing bowl prepare the crumb by mixing together the light brown sugar, flour and cinnamon. Stir to combine. Then using a fork mix until large crumbs are formed. Set aside.
In a large mixing bowl add the room temperature butter, white and light brown sugars. Cream together. Add in the eggs and vanilla extract. Stir to combine.
Mash the bananas with a fork, then add to the batter, stir to combine, will be slightly lumpy.
In a medium size mixing bowl combine the dry ingredients: flour, baking powder, baking soda, salt, ground cinnamon, stir with a fork to mix.
Add to the wet ingredients, stir slowly and pour in the buttermilk while stirring. Stir until just combined.
Pour the batter into the prepared baking pan and sprinkle the crumbs on top. Gently press the crumbs into the batter to help stick.
If using an 8x8 pan bake for about 45-50 minutes. If using a 9x9 pan bake for about 35-40 minutes.
Allow to cool for about 30 minutes prior to serving. If desired top with icing.
If you don't have buttermilk on hand you can make sour milk. Add 2 teaspoons of lemon juice or plain white vinegar to your measuring cup, then pour the milk in to reach 2/3 cup. Let it sit briefly. This will make sour milk.If you want to make this in a 9x13 pan you can add an additional 1/2 cup flour and 1/3 cup milk to the recipe. Double the crumb recipe and bake for 40 minutes.