Preheat the oven to 350° F. Grease an 8×8-inch baking dish with baking spray. Set aside.
In a medium bowl, combine the mashed bananas, light brown sugar and cooled butter. Stir in egg, buttermilk, and vanilla extract.
In a separate bowl, whisk the flour, baking powder, and cinnamon together until combined. Stir these dry ingredients into the wet ingredients just until combined. Don’t over mix.
Spread the batter evenly into the prepared baking dish. Sprinkle the cinnamon topping all over the top of the batter. Using a butter knife, gently swirl the topping into an inch of the top of the batter. Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out mostly clean. Remove from the oven and allow to cool for 30 minutes.