In a medium bowl, mix together chocolate crumbs and melted butter. Press into a 9-inch springform pan.
Preheat oven to 300 degrees.
In a large bowl, using an electric mixer set on medium speed, beat together cream cheese, sugar and eggs until fluffy. Spoon half the cream cheese mixture into the crust.
Stir 2 oz. chocolate into remaining cream cheese mixture until well blended. Drizzle over batter in crust to make swirls.
Bake for 50 minutes. Transfer the pan to a wire rack. Cool the cheesecake completely.
Transfer the cheesecake to a serving plate, cover with plastic wrap and chill in the refrigerator at least 2 hours.
Uncover the cheesecake; carefully remove the side of pan.
In a small bowl, mix together the 6 ounces chocolate and sour cream. Spread over top of cheesecake. Chill briefly until frosting is set.
Cheesecake can be frozen up to 3 weeks. Defrost before serving.