Preheat oven to 325 degrees. Combine over hot (not boiling) water, chocolate chips and 1/2 cup sugar; heat until chips melt and mixture is smooth. Remove from heat; set aside.
In a small bowl combine graham cracker crumbs, 2 tablespoons sugar and melted butter; mix well. Pat into a 9 inch springform pan, covering bottom and 1 1/2 inch up sides of pan.
In large bowl, beat cream cheese until light and creamy. Gradually beat in 3/4 cup sugar. Mix in sour cream and vanilla. Add eggs, one at a time, beating well after each addition.
Divide batter in half. Stir melted chocolate mixture into first half. Pour into crumb lined pan; cover with plain batter. With a knife, swirl plain batter with chocolate batter to marbelize. Bake at 325 degrees for 50 minutes or until only a 2-3 inch circle in center will shake. Cool at room temperature; refrigerate until ready to serve. Makes one 9 inch cheesecake.
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