Grease bottom of 9-inch springform pan. Prepare cake mix as directed on package; pour batter evenly into prepared springform pan. Bake at 350 for 20 minutes.
Beat cream cheese, granulated sugar and 1 teaspoon of the vanilla with electric mixer on medium speed until well blended. Add eggs, 1 at a time, mixing on low speed after each addition just until blended. Blend in sour cream; pour over cake layer.
Bake an additional 35 minutes or until center is almost set. Run a knife or metal spatula around rim of pan to loosen cake; cool before removing rim of pan.
Melt chocolate and butter in medium saucepan over low heat, stirring until smooth. Remove from heat. Add water, powdered sugar and remaining 1 teaspoon vanilla; mix well. Spread over cooled cheesecake. Refrigerate 4 hours or overnight.
TIP: If you find it is too hard to spread the chocolate mixture over the cheescake..try drizzling it on with a Decorator's bag or with a spoon.