Preheat oven to 350°. Spray a 13×9-inch baking pan with nonstick spray. Prepare brownie batter according to packaged directions using 2 eggs, oil, and water; set aside.

Whisk together flour, sugar, baking soda, salt, and cinnamon in a medium bowl; set aside. Whisk remaining 1 egg in a large bowl; stir in mashed bananas, melted butter, sour cream, and vanilla. Add flour mixture; stir just until combined.

Dollop half of the banana batter, by the spoonful, onto the bottom of the prepared baking pan. Pour brownie batter around the banana batter. Dollop remaining banana batter over the top, allowing some brownie batter to show. Gently swirl batters with tip of knife.

Bake 35 to 40 minutes or until a toothpick inserted near the center comes out clean. Cool completely in pan on wire rack.

Microwave frosting in a medium microwave-safe bowl on HIGH 20 seconds or until softened. Spread frosting evenly over brownies. Garnish with jimmies and/or nonpareils, if desired. Let stand 30 minutes or until frosting is set.