Prepare brownie batter as directed on package; stir in 1/2 cup sour cream. Spoon into greased and floured 9-inch round pan.
Bake 45 minutes; cool 10 minutes. Remove from pan to wire rack; cool completely.
Mix remaining sour cream, COOL WHIP, sugar and vanilla. Cut brownie horizontally in half. Stack layers on plate, filling with half the sour cream mixture. Top with remaining sour cream mixture and fruit.