Preheat oven to 350 degrees and line 1/2 sheet baking pan with a silpat liner or parchment paper, sprayed with cooking spray.
Place cake mix, egg, butter and mashed bananas into a large mixing bowl. Stir to combine then stir in chocolate chips. Transfer batter to prepared baking sheet and spread evenly almost to edges. Bake for 25-35 minutes, or until baked through. Remove from oven and let cool for 30 minutes.
Place powdered sugar, cream and syrup into a large bowl, whisk until smooth and desired consistency. You may add additional cream and or syrup to thin out more if needed. Spread over cake evenly and top with sprinkles if desired. Cut into squares and serve.