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PREHEAT oven to 350°F. Line 13x9-inch baking pan with foil, with ends of foil extending over sides of pan. Lightly grease foil; set aside. Beat ricotta cheese, sugar, eggs, cornstarch and vanilla in large bowl with electric mixer on medium speed until well blended.
PREPARE brownie batter as directed on package. Spread 2/3 of the brownie batter into prepared pan; cover with the ricotta mixture. Drop remaining brownie batter by spoonfuls over ricotta mixture. Cut through batter with knife several times for marble effect.
BAKE 50 minutes or until toothpick inserted in center comes out clean. Cool in pan. Remove dessert from pan using foil handles. Cut into 32 brownies to serve.