Preheat oven to 350℉. Line 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. Drizzle with the melted butter, then use 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 the dry ingredients 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.