1 cup all-purpose flour, divided
1 cup plus 2 tablespoons granulated sugar
1/4 teaspoon salt
1/2 cup butter
3 (1 ounce) packets pre-melted unsweetened chocolate
24 ounces cream cheese, softened
2 teaspoons vanilla extract
1 cup sour cream
Preheat oven to 400 degrees F.
Combine 3/4 cup of the flour, 2 tablespoons of the sugar and salt. With pastry blender or fork cut butter into flour mixture until fine; stir in 1 packet chocolate. Press mixture in bottom of a 9-inch springform pan. Bake 10 minutes. Remove from oven; turn oven to 475 degrees F.
In large bowl beat at low speed cream cheese and remaining 1 cup sugar until smooth; blend in remaining 1/4 cup flour and vanilla extract. Add eggs one at a time, beating well after each. Beat in sour cream.
In small bowl combine 1 1/2 cups cheese mixture with remaining 2 packets chocolate. Pour 1/2 the plain cheese mixture over crust. With circular motion, drizzle half the chocolate mixture on top. Repeat sequence. With a spatula cut through mixture to marbleize it. Bake at 475 degrees F for 12 minutes; turn oven down to 200 degrees F and bake 1 hour longer.
Remove from oven. Cool for 2 to 3 hours. Refrigerate for 8 hours. To cut, use sharp knife which has been run under hot water. Cracks in top are characteristic. Should be made the day before.