Preheat oven to 350. Prepare brownie mix to package directions. Spread into well greased 13 x 9 inch baking pan. In small mixer bowl, beat peanut butter, cream cheese, butter and cornstarch until fluffy. Gradually beat in sweetened condensed milk, egg and vanilla until smooth. Pour evenly over brownie batter. Bake 45 minutes or until top is lightly browned.
Cool and chill thoroughly. Garnish as desired and cut into bars.
I garnish mine by drizzling melted chocolate or some lightly sifted cocoa powder.