Heat oven to 300. Crush chocolate wafers in food processor or in plastic bag with rolling pin. Add butter and process or knead into crumbs. Pat into bottom of an oiled 23 cm spring-form pan. Bake 10 minutes.

In the large bowl of an electric mixer, beat cream cheese until fluffy. Stir together sugar and cornstarch. Beat into cream cheese mixture.

Add eggs to mixture one at a time, beating after each addition. Fold in vanilla, chocolate, and sour cream until uniformly combined.

Turn cheesecake mixture into hot, crumb-lined pan. Carefully return to oven. Bake until center of cake seems to be almost set when the pan is gently tapped-about 1 hour. Turn off oven and allow cake to sit in oven 1 hour longer.

Remove pan from oven; cool to room temperature. Cover without touching surface of cake. Place in refrigerator overnight.

To serve, loosen cake from side of pan with a spatula. Remove side of pan. Place cake on serving platter. Decorate with whipped cream, if desired.