1. Preheat oven to 325 degrees
2. 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.
3. In small bowl, combine graham-cracker crumbs, 2 tablespoons sugar and melted butter; mix well. Pat firmly into 9" springform pan, covering bottom and 1//2 inch up sides; set aside.
4. In large bowl, beat cream cheese until light and creamy. Gradually beat in 3/4 cup sugar. Mix sour cream and vanilla extract. Add egge, 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 knife, swirl plain batter with chocolate batter to marbleize.
5. Bake for 50 minutes or until only 2-3 inch circle in center will shake. Cool at room temperatute; refrigerate.