Beat brownie mix, melted butter, cream cheese and egg in medium bowl 50 strokes with spoon until well blended (dough will be sticky).
Drop dough by rounded tablespoonfuls 2 inches apart onto ungreased cookie sheets to make 24 cookies; smooth edge of each to form round cookie.
Mix powdered sugar and peanut butter in small bowl with spoon until mixture forms a ball. With hands, roll rounded teaspoonfuls peanut butter mixture into 24 balls. Lightly press 1 ball into center of each ball of dough.
Bake 10 to 14 minutes or until edges are set. Cool on cookie sheets at least 30 minutes.
Spread thin layer of frosting over peanut butter portion of each cooled cookie.