PREHEAT oven to 350°F. Prepare brownie batter as directed on package; stir in 1/2 cup sour cream. Spoon into greased and floured 9-inch round cake pan.
BAKE 45 min.; cool 10 min. Remove from pan to wire rack; cool completely.
MIX remaining sour cream, whipped topping, sugar and vanilla. Cut brownie horizontally in half. Place bottom half on plate; spread with half of the sour cream mixture. Cover with top of brownie, remaining sour cream mixture and fruit. Store in refrigerator.