1Heat oven to 350°F. Make and bake brownies in 13x9-inch pan as directed on box. Cool completely. Cut into 6 rows by 4 rows, making 24 brownies.
2Divide frosting among 3 small microwavable bowls. Heat frosting as needed: microwave 1 bowl uncovered on High about 10 to 15 seconds until frosting can be stirred smooth. If frosting becomes too firm while decorating, microwave on High an additional 5 seconds; stir.
3Decorate as directed below, using 8 brownies for each bowl of frosting.