HEAT oven to 350ºF.
PREPARE brownie batter as directed on package; spread into greased 13x9-inch pan.
BEAT cream cheese with mixer until creamy. Add sugar, egg and vanilla; mix well. Drop by tablespoonfuls over brownie batter; swirl with knife.
BAKE 35 to 40 min. or until cream cheese mixture is lightly browned. Cool completely before cutting to serve. Keep refrigerated.