Make brownie batter as directed on box. Stir in peanuts. Spread in pan. Place 24 Reese's™ peanut butter cups in single layer 6 rows by 4 rows on top of batter. Bake 35 to 38 minutes or until toothpick inserted 2 inches from side of pan comes out almost clean. Transfer to cooling rack to cool completely, about 1 hour.