Preparation Preheat oven to 350°F. Spray 9-inch-diameter cake pan with 2-inch-high sides with nonstick spray. Puree 1 cup grapes and grape juice concentrate in blender until as smooth as possible. Pour puree into medium saucepan. Add 1/4 cup sugar and cornstarch; stir to dissolve cornstarch. Add remaining 2 1/4 cups grapes. Stir over medium heat until mixture thickens, boils, and becomes clear, 2 to 3 minutes (grapes will remain whole); mix in vinegar. Pour into prepared cake pan. Spread grapes in single layer; cool. Beat cream cheese, flour, and 3/4 cup sugar in large bowl until smooth. Beat in mascarpone, then eggs, 1 at a time. Spoon batter over grape mixture. Bake cake until just set in center, about 45 minutes. Place cake directly into refrigerator; chill uncovered until cold, at least 4 hours and up to 1 day. Cut around cake. Place plate over pan; hold plate and pan together and invert. Lift off pan. Cut cake into wedges. An Italian cream cheese; sold at many supermarkets and at Italian markets.
Preparation Preheat oven to 350°F. Spray 9-inch-diameter cake pan with 2-inch-high sides with nonstick spray. Puree 1 cup grapes and grape juice concentrate in blender until as smooth as possible. Pour puree into medium saucepan. Add 1/4 cup sugar and cornstarch; stir to dissolve cornstarch. Add remaining 2 1/4 cups grapes. Stir over medium heat until mixture thickens, boils, and becomes clear, 2 to 3 minutes (grapes will remain whole); mix in vinegar. Pour into prepared cake pan. Spread grapes in single layer; cool. Beat cream cheese, flour, and 3/4 cup sugar in large bowl until smooth. Beat in mascarpone, then eggs, 1 at a time. Spoon batter over grape mixture. Bake cake until just set in center, about 45 minutes. Place cake directly into refrigerator; chill uncovered until cold, at least 4 hours and up to 1 day. Cut around cake. Place plate over pan; hold plate and pan together and invert. Lift off pan. Cut cake into wedges. An Italian cream cheese; sold at many supermarkets and at Italian markets.