Preheat oven to 350 degrees F. Grease an 8x8x2-inch baking pan; set aside. In a medium bowl combine flour, sugar, cocoa powder, baking powder, baking soda, and salt; set aside. In a blender combine the water, blueberries, and egg. Cover and blend until smooth. Add blueberry mixture to flour mixture. Whisk until well combined. Pour into prepared pan, spreading evenly.
Bake for 30 minutes or until a toothpick inserted in the center comes out clean. Place cake in pan on a wire rack; cool completely. Invert onto a serving platter.
In a small microwave-safe bowl combine dessert topping and chocolate pieces. Microwave, covered, on 50 percent power (medium) for 1 minute. Stir until smooth. Let stand for 5 minutes. Pour onto cooled cake, spreading evenly. Serve cake with Blueberry Sauce, fresh blueberries, and, if desired, mint leaves.
In blender combine dessert topping and blueberries. Cover and blend until smooth.