Grease bottom of 9-inch springform pan. Prepare cake batter as directed on package; pour into prepared pan.
Bake at 325°F for 25 minutes if using a silver springform pan. (Bake at 300°F for 20 minutes if using a dark nonstick springform pan.) Cool.
Beat cream cheese, granulated sugar and 1 teaspoon of the vanilla in large bowl with electric mixer on medium speed until well blended.
Add sour cream; mix well.
Add eggs, 1 at a time, mixing on low speed after each addition just until blended. Pour over cake layer in pan.
Bake at 325°F for 40 to 45 minutes or until center is almost set if using a silver springform pan. (Bake at 300°F for 40 to 45 minutes or until center is almost set if using a dark nonstick springform pan.).
Run knife or metal spatula around rim of pan to loosen cake; cool before removing rim of pan.
Place chocolate, milk and butter in medium microwavable bowl.
Microwave on HIGH 2 minutes or until butter is melted, stirring after 1 minute. Stir until chocolate is completely melted.
Add powdered sugar and remaining 1 teaspoon vanilla; mix well.