Preheat oven to 160ºC . Combine over hot (not boiling) water, chocolate chips and heavy cream. Stir until morsels are melted and mixture is smooth. Set aside. In large bowl, combine cream cheese and sugar, beating untl creamy. Add eggs, one at a time, beating well after each addition. Add chocolate mixture and vanilla, mix until blended. Pour into prepared crust. Bake at 160ºC . for 60 minutes. Turn oven off. Let stand in oven with door ajar 1 hour. Remove, cool completely. Chill 24 hours. Decorate edge with whipped cream, if desired.