1Heat oven to 325°F. Grease bottom only of 13x9-inch pan with shortening or cooking spray. Make brownie batter as directed on box. Spread three-fourths of brownie batter in pan. Set remaining batter aside.
2In small bowl, mix brown sugar, 1/2 cup crunchy peanut butter and 1 egg. Spoon peanut butter mixture by tablespoonfuls evenly onto brownie batter. Top with random tablespoonfuls of remaining brownie batter. Cut through batter several times with knife for marbled design.
3Bake 33 to 38 minutes or until toothpick inserted 2 inches from side of pan comes out clean or almost clean. Cool completely, about 1 1/2 hours.
4In medium bowl, mix Peanut Butter Frosting ingredients, using spoon, until smooth and spreadable. Spread frosting over brownies. For brownies, cut into 6 rows by 5 rows. Store tightly covered.