Preparation / Directions:
Preheat oven to 325 degrees. Butter a 9-inch springform pan, line it with wax paper or parchment paper and butter paper and sides of pan. In a large bowl with an electric mixer cream together cream cheese and sugar. Beat in eggs, one at a time, beating well after each addition. Add butter, flour, cornstarch and vanilla and beat mixture until well combined. Fold in sour cream. Pour two-thirds of batter into prepared pan. To remaining batter add chocolate and gently whisk mixture until combined. Pour chocolate batter onto white batter and swirl a spatula lightly through mixture to create a marbled pattern. Bake cake for 1 hour (it will be soft in center). Turn off heat (do not open oven door) and let cake stand in oven for 2 hours. Remove cake from oven, set it on a rack, and let it cool completely in pan. Chill cheesecake, loosely wrapped, for at least 2 hours. Remove sides of pan and transfer cake to a cake plate.