To cut clean slices, dip a sharp knife into water before each cut.
Wrap the outside bottom and sides of a 9-inch spring-form pan with heavy-duty aluminum foil. Spray the bottom of the pan heavily with nonstick. Mix the crushed cookies and the melted butter. Press the crumb mixture into the bottom of the pan to form a crust.
In a large mixing bowl combine the sugar, flour, vanilla, cream cheese and sour cream. Beat at medium speed with an electric mixer until smooth. Add the eggs, 1 at a time; beating well after each addition.
Pour 1 1/2 cups of the mixture into a bowl; add melted and cooled chocolate, stirring until well blended.
Pour the remaining vanilla cream cheese mixture into the pan. Spoon the chocolate cream cheese mixture over vanilla cream cheese mixture; swirl with a knife to create a marbled effect.
Place the pan in a larger shallow pan; add hot water to larger pan to a depth of 1 inch. Bake at 325 degrees for 45 minutes; remove from oven. Carefully remove the spring-form pan from water and remove the foil from the pan.
Carefully run knife around edge to loosen the cheesecake from the pan; leave side of pan fastened. Let cool for 1 hour. Cover and chill in the refrigerator for at least 4 hours or overnight before serving.