1. Heat oven to 300°F. Line a 13 x 9-inch pan with foil so that the foil extends about 2-inches over the sides of the pan, then spray the foil with cooking spray.
2. Make brownies as directed on box for cakelike brownies in a 13 x 9-inch pan and cool completely, about 1-1/2 hours. Remove from the pan by lifting the foil and then peel the foil away. Cut the brownie crosswise into thirds; place one piece on serving plate.
3. Spoon the frosting into a microwavable bowl and microwave uncovered on High for 15 to 20 seconds. Stir the frosting until smooth.
4. Pour 1/3 of the frosting over the brownie piece on the serving plate. Place another brownie piece on top and pour half of the remaining frosting over second brownie piece. Top with remaining brownie piece and pour the remaining frosting over the top. Refrigerate for 30 minutes to set. Garnish with chocolate curls or peanut butter cup crumbles and store loosely covered.