Preheat the oven to 325°F. In a bowl, mix the chocolate cookie crumbs and melted butter until well combined. It should be the consistency of wet sand. If dry, add a little more melted butter at a time until it reaches the right texture.

Firmly press the mixture into a 9-inch springform pan. Bake for 10 minutes, then cool.

In a large bowl, beat the softened cream cheese until smooth. Add the sugar and cocoa powder, and mix until blended.

Add eggs and yolks one at a time, beating after each addition. Stir in vanilla extract and sour cream until smooth. Gently fold in melted chocolate until evenly distributed.

Pour the filling over the cooled crust. Bake at 325°F (163°C) for 55 to 65 minutes or until the center is almost set. The center should still jiggle slightly. Remove from the oven and run a knife around the edge to loosen. Let it cool to room temperature.

In a saucepan, melt the semi-sweet chocolate chips with heavy cream over low heat, stirring until smooth. Pour this chocolate mixture over the cooled cheesecake, ensuring it is spread evenly.

Refrigerate the cheesecake for at least 4 hours or overnight. Before serving, remove the sides of the springform pan. Slice and enjoy.