For bottom layer: Heat oven to 350 degrees. Grease bottom of a 9-inch springform pan. Prepare cake mix as directed on package; pour half the batter evenly into greased springform pan. Bake 20 minutes.
    (Suggestion: Use extra batter to make a plain cake in a small round pan, as it won’t be needed for the rest of the recipe.)
    For filling: Beat cream cheese, sugar and vanilla with electric mixer until well blended. Add eggs one at a time, mixing at low speed after each addition just until blended. Add in sour cream; pour cream-cheese mixture over cake.
    Bake 35 minutes or until center is almost set. Run knife around rim of pan to separate cake from side of pan.
    For topping: Melt chocolate and margarine over low heat, stirring until smooth. Remove from heat. Add water, sugar and vanilla; mix well. Spread over cooled cheesecake. Refrigerate at least 4 hours or overnight.