Preheat oven to 350 degrees F. Liberally grease a 13x9" baking dish with cooking spray and set aside. In a large bowl, beat together the cake mix, brown sugar, mashed banana, cinnamon, water, oil, eggs and banana extract with a handheld electric mixer for about 2 minutes or until blended.
Pour the batter into the prepared pan and bake for approx. 30-35 minutes or until a toothpick inserted near the center comes out clean or with moist crumbs. Cool the cake completely.
Once the cake has cooled, prepare your frosting. In the bowl of a stand mixer, cream together the butter, mashed banana, vanilla, banana extract and lemon juice with the paddle attachment until creamy. Add in the powdered sugar, about a cup at a time, until the frosting is light and fluffy but still kind of soft. You don't want it as stiff as you normally would for say, frosting cupcakes, but it should hold its shape relatively well.
Spread the frosting on top of the cake in an even layer. Refrigerate the cake for at least 30 minutes before cutting into squares and serving. Store leftovers airtight in the fridge for up to 4 days.