1. Heat oven to 350°F. Line 13x9x2-inch baking pan with foil, extending foil beyond pan sides. Lightly grease foil.
2. Stir together cake mix, cocoa and 1/4 cup sugar; set aside 1/2 cup cake mixture. Mix remaining cake mixture with melted butter, 1 tablespoon red food color, water and 1 egg until dough forms. Stir in chocolate chips. Press dough evenly on bottom of prepared pan.
3. Beat together cream cheese, remaining 1/2 cup sugar and vanilla until well blended. Beat in sour cream and remaining 2 eggs. Set aside 1/2 cup cream cheese mixture; pour remaining mixture in crust, spreading evenly.
4. Beat reserved cake mixture, reserved cream cheese mixture, remaining 1 tablespoon red food color and milk until well blended. Drop by tablespoons onto vanilla batter in pan. Swirl with knife for marbled effect.
5. Bake 30 to 35 minutes or until center is set. Cool completely in pan on wire rack. Cover; refrigerate until chilled. Cut into squares. Refrigerate leftovers. Makes 24 squares.