Preheat oven to 325 degrees F. Grease the bottom of a 9-inch springform pan; set aside. Prepare cake mix according to package directions, except add cooking oil. Pour batter into the prepared pan, spreading evenly. Bake for 25 minutes.
Meanwhile, in a large bowl, combine cream cheese, sugar, and vanilla. Beat with an electric mixer on medium speed until smooth. Add eggs; beat on low speed just until combined. Stir in sour cream. Carefully pour cream cheese mixture over cake layer.
Bake for 50 to 55 minutes or until a 2-inch area around the outside edge appears set and the center appears nearly set when gently shaken. Cool in pan on a wire rack for 15 minutes. Using a small sharp knife, loosen edge of cheesecake from side of pan. Cool completely (about 1 3/4 hours). Remove side of pan; transfer cheesecake to a serving plate. Cover and chill for 4 to 24 hours.
Before serving, in a small saucepan, bring whipping cream to simmering. Remove from heat. Add chocolate; stir until chocolate is melted and mixture is smooth. Stir in butter. Chill about 15 minutes or until mixture is slightly thickened, stirring once. Spoon chocolate mixture onto cheesecake, spreading to cover top. Makes 16 servings.