1Heat oven to 300°F. Wrap outside bottom and side of 10-inch springform pan with heavy-duty foil. Spray inside of pan with baking spray with flour. Reserve 1/4 cup of the cake mix for filling; set aside. In large bowl, beat remaining cake mix and butter with electric mixer on low speed. Press in bottom and 1 1/2 inches up side of pan.
2In large bowl, beat reserved 1/4 cup cake mix, the cream cheese, chocolate, sour cream, sugar and food color with electric mixer on medium speed until smooth. Beat in eggs, one at a time, just until blended. Pour over crust.
3Bake 1 hour 5 minutes to 1 hour 15 minutes or until edge of cheesecake is set at least 2 inches from edge of pan but center of cheesecake still jiggles slightly when moved. Turn off oven; open oven door 4 inches. Leave cheesecake in oven 30 minutes. Remove from oven and cool in pan on cooling rack 30 minutes. Refrigerate 6 hours or overnight. Run small metal spatula around edge of pan; remove side of pan. Pipe whipped topping around outer edge of cheesecake. Store covered in refrigerator.