1Heat oven to 350°F (325°F for dark or nonstick pan). Line bottom and sides of 13x9-inch pan with foil. Grease bottom only of foil with shortening or cooking spray. Make brownie mix as directed on box for 13x9-inch pan. 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 or just until melted and spoonable, but still thick. DO NOT BOIL. Carefully spoon melted frosting over brownies to glaze, allowing excess to drip down sides. Sprinkle with candy sprinkles before frosting sets. Allow frosting to set, about 1 hour.