Heat oven to 300. Line 13x9-inch pan with foil so foil extends about 2"-inches over sides of pan. Spray foil with cooking spray.
Make brownies as directed on box for cakelike brownies in a 13x9"-inch pan. Cool completely, about 1 1/2 hours. Remove from pan by lifting foil; peel foil away. Trim sides if desired. Cut brownie crosswise into thirds; place one piece on serving plate.
Spoon frosting into microwavable bowl. Microwave uncovered on HIGH 15-20 seconds; stir frosting until smooth.
Pour 1/3 of the frosting over brownie piece on serving plate. Place another brownie piece on top; pour half of remaining frosting over second brownie piece. Top with remaining brownie piece; pour remaining frosting over top. Refrigerate 30 minutes to set. Garnish with chocolate curls. Store loosely covered.