CAKE BOTTOM - Grease bottom of 9-inch springform pan. Prepare cake mix as directed on package. Pour batter into the springform pan. Bake 20 minutes at 350°.
FILLING - Beat cream cheese, sugar, and vanilla at medium speed with an electric mixer until well blended. Add the eggs, 1 at a time, mixing at low speed after each addition, just until the mixture is well blended. Blend in the sour cream. Pour the batter over the cake layer.
Place pan in oven and bake at 350° for 35 minutes or until the center is almost set. Run a butter knife or metal spatula around the rim of the pan to loosen the cake. Cool before removing rim of pan.
TOPPING - Melt chocolate and butter in a small saucepan over low heat, stirring until smooth. Remove from heat. Add the boiling water, powdered sugar and vanilla; mix well. Spread the chocolate mixture over the cooled cheesecake.