Preheat oven to 350 degrees F. Line an 8x8 square baking dish with parchment paper. Lightly spray with cooking spray and set aside. Prepare the bars. In a large mixing bowl, combine butter, light brown sugar, egg and vanilla. Mix until creamy.
In a medium bowl, combine flour, baking powder, salt and cinnamon. Mix well and gradually add to batter. Mix until incorporated. Add mashed bananas and stir until combined. Pour into prepared baking dish. Spread batter evenly in pan with a spatula and bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean. Let cool.
While the bars are baking, prepare the frosting. In a mixing bowl, combine the butter, powdered sugar, dark brown sugar, salt and vanilla. Mix well. Gradually pour in the maple syrup while mixing. Mix on medium speed until frosting is fluffy and smooth.
When bars are cooled, spread frosting evenly over the top. Refrigerate until ready to serve and cut into small squares. Store in a sealed container in the refrigerator.