Start by preparing the crust: Thoroughly blend the chocolate crumbs and butter together. Press onto the bottom and sides of a 23 cm springform pan. Preheat the oven to 300 degrees. Prepare the filling next: Bring the cream cheese fully to room temperature. Beat the cream cheese, sugar and eggs, using medium speed on a heavy duty mixer, until the mixture is smooth, free of lumps and fluffy. Spoon half the mixture into the crust. Stir the melted chocolate into the remaining filling. Blend thoroughly. Drizzle over the filling already in the crust to form swirls. Bake for 50 minutes. Cool completely on a wire rack. Transfer the fully cooled cake to a serving plate. Cover with plastic wrap. Refrigerate for at least 2 hours. Uncover the cake. Remove the sides of the springform pan. Prepare the frosting by combining the melted chocolate and the sour cream in a small bowl. Spread over the top and sides of the cake. Refrigerate briefly to set the frosting.