1Heat oven to 350°F. Place paper baking cup in each of 12 regular-size muffin cups. Make brownie batter as directed on box. Fill cups about 3/4 full (about 1/4 cup each).
2Bake 25 to 28 minutes or until toothpick inserted near edge comes out almost clean. Cool 5 minutes; remove from pan. Cool completely.
3In small bowl, mix all frosting ingredients until smooth and spreadable. Spread frosting over brownies; sprinkle with candy sprinkles. Store tightly covered.