1Heat oven to 350°F. Line bottom and sides of 13x9-inch pan with foil. Grease foil on bottom only of pan with shortening or cooking spray. Make and bake brownie mix as directed on box. Cool completely, about 1 hour.
2Using foil to lift, remove brownie from pan; remove foil. With deep 2 1/2-inch heart-shaped cookie cutter, cut 14 brownies. Place brownie hearts on a cooling rack or waxed paper.
3In microwavable bowl, microwave frosting uncovered on High 10 to 30 seconds. Stir; frosting will be thick and spoonable. DO NOT BOIL. Carefully spoon melted frosting over brownies to glaze, allowing excess to drip down sides. Sprinkle with candy sprinkles before frosting sets.