Heat oven to 350°F (325°F for dark or nonstick pan). Line 9-inch square pan with foil, allowing some to hang over edges of pan. Grease bottom and sides of foil with shortening or cooking spray. Make and bake brownies as directed on box. Cool completely on cooling rack, about 1 1/2 hours. Freeze in pan 30 minutes.
Using foil to lift, remove brownies from pan, and peel foil away. With deep 2 1/2-inch heart-shaped cookie cutter, cut 9 brownies. Set aside leftover pieces for snacking.
Place brownie hearts on a cooling rack or waxed paper.
In 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 2 hours. Store covered in a single layer at room temperature.
