To prepare crust, in a medium bowl, mix together chocolate crumbs and melted butter until well blended. Press into a 9-inch spring form pan. Preheat oven to 300 F. To prepare filling, in a large bowl, beat together cream cheese, sugar, and eggs at medium speed until smooth and fluffy. Spoon half of cream cheese mixture into crust. Stir chocolate into remaining cream cheese mixture until well blended. Drizzle over batter in curst to make swirls. Bake cheesecake for 50 minutes. Transfer pan to a wire rack. Cool completely. Transfer cheesecake to a serving plate, cover with plastic wrap, and chill for 2 hours. Uncover cheesecake; carefully remove the side of pan. To prepare frosting, in a small bowl, mix together chocolate and sour cream. Spread over cheesecake. Chill briefly until frosting is set.