Preheat oven to 325. line a baking sheet with foil. Prepare a 10 inche springform pan by spraying with nonstick spray and lining bottom with parchment paper cut to fit and spraying the top of the paper.

In a large bowl combine cream cheese with sugar and beat until smooth, add sour cream and mix in, add eggs and beat on low just until combined.

Remove 3 1/2 cups batter to another bowl,gently stir in melted chocolate. Stir vanilla extract into remaining white batter.

Pour chocolate batter into prepared springform pan.Spread evenly,carefully pour vanilla batter over chocolate layer taking care not to mix flavors.

Put cake on foil lined baking sheet, bake at 325 for 65 to 75 minutes or until center is just set.

Cool on a wire rack for 10 minutes, carefully run knife around edge of pan to loosen, cool another hour on rack in pan.

FOR TOPPING

In a small saucepan bring cream to a simmer.pour over chopped chocolate, let sit a minute and stir until smooth. Cool slightly until just warm enough to still be able to pour. Gently remove sides of springform pan and pour chocolate topping on top of cheesecake.

Refrigerate overnight.