HEAT oven to 350°F.
PREPARE brownie batter as directed on package. Add 1/2 cup peanut butter; mix well. Pour into greased and floured 9-inch round pan.
BAKE 35 to 40 min. or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Cool in pan 10 min. Invert brownie onto wire rack; gently remove pan.
BEAT cream cheese, remaining peanut butter and sugar in large bowl with mixer until blended. Gently stir in COOL WHIP; spread onto brownie.