Prepare the pan. Preheat the oven to 350°F. Line an 8×8-inch baking pan with parchment paper and lightly coat it with cooking spray.
Mix the wet ingredients. In a large mixing bowl, beat the softened butter, light brown sugar, egg and vanilla until creamy.
Add the dry ingredients. In a separate bowl, whisk together the flour, baking powder, salt and cinnamon. Gradually add the dry ingredients to the wet ingredients, mixing just until incorporated.
Add the bananas. Stir in the mashed bananas until evenly combined. Spread the batter in an even layer inside the prepared pan.
Bake and cool. Bake for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs. Let the bars cool completely.
Make the frosting. Beat the softened butter, powdered sugar, dark brown sugar, salt and vanilla until combined. Gradually pour in the maple syrup while mixing. Continue beating on medium speed until the frosting is fluffy and spreadable.
Frost the bars. Spread the frosting over the completely cooled bars. Refrigerate until the frosting is set, then cut into 12 squares.