1Heat oven to 350°F (325°F for dark or nonstick pan). Grease bottom only of 13x9-inch pan with shortening or cooking spray. In small bowl, beat sugar, 2/3 cup peanut butter, flour and 2 eggs with electric mixer on medium speed 2 minutes; set aside.
2Make brownie batter as directed on box. Spread in pan. Drop peanut butter mixture by tablespoonfuls onto batter. Swirl through batter with knife.
3Bake 29 to 32 minutes or until toothpick inserted 2 inches from side of pan comes out almost clean. Cool completely, about 1 1/2 hours.
4In small bowl, stir all frosting ingredients, using spoon, until smooth and spreadable. Spread frosting over brownies; sprinkle with peanuts. For brownies, cut into 8 rows by 4 rows.