Combine first four ingredients and press into the bottom of a 8-inch springform pan. Beat eggs and sugar with electric beater until a pale mixture. Mix well. Melt chocolate in double boiler. Stir in sour cream, butter and vanilla. Add to cheese mixture and stir. Pour batter into pan. Bake at 325F for 2 hours or until centre is firm. Cool on wire rack. Remove from pan. Chill and serve with whipped cream.
