Make the crust. Preheat the oven to 350 degrees F (180C). Assemble and grease a 9 inch (22cm) springform pan, line with parchment and grease again. Set aside.
Put the cookies in the bowl of a food processor fitted with a steel blade attachment. Pulse until finely ground. Add the melted butter and pulse again until just coated with butter. Pour mixture into the prepared springform pan and, using the tines of a fork, press the ground cookies into the bottom and up the sides of the pan, about ¾ of the way up.
Bake the crust in the preheated oven for 15 minutes. Remove from the oven and cool. Reduce oven temperature to 300 degrees F (150C).
Meanwhile, prepare the filling. Beat the cream cheese in the bowl of a stand mixer fitted with a paddle attachment on low speed until it’s smooth. Add the sugar, prune purée, cocoa powder, cream cheese, eggs, and melted chocolate and beat again, on low speed, until the mixture is totally incorporated. Stir in the vanilla extract and sea salt. Pour the cream cheese mixture into the cooled crust and smooth the top with an offset spatula (or rubber spatula).
Prepare a water bath. Wrap the pan bottom and sides tightly in heavy-duty aluminum foil. Place the springform pan within a larger pan that leaves at least a 1-inch (2.5cm) gap around all sides. A large cake pan or roasting pan works well for this. Carefully pour hot water around the sides of the cheesecake until it’s about halfway up the sides of the pan. Transfer the cheesecake in its water bath into the oven and bake for about 1 hour, or until the middle of the cheesecake is just set. It will still be a little jiggly. Remove the cheesecake from the oven and take it out of the water bath. Carefully peel off the foil and return the cheesecake to the oven. Turn the oven off, and leave in the oven for an additional 30 minutes. Finally, crack the oven door, and cool in the oven for another 30 minutes. These steps cool the cheesecake slowly and help prevent it from cracking.
Remove the cheesecake from the oven and cool at room temperature until it’s barely warm, then transfer to a refrigerator and cool for 3-6 hours, loosely covered.
Make the ganache for the swirls. The ganache for the decorative swirls should be made first, and cooled to room temperature before using. Heat the heavy cream until it just comes to a boil and pour over the chocolate chips in a medium bowl. Stir the mixture until the chocolate melts and is totally smooth. Cover and let cool to room temperature. It will be thick and pipeable at that point. If you don’t want to pipe swirls on the cheesecake you can obviously skip this step.
Make the ganache for the topping. Heat the heavy cream until it just comes to a boil and pour over the chocolate chips in a medium bowl. Stir the mixture until the chocolate melts and it’s totally smooth. Pour over the top of the cooled cheesecake and tilt or spread with an offset spatula to cover the entire top of the cheesecake. Return it to the fridge for 15 minutes to set.
Snip off the corner of a piping bag and fit with a large star tip. Fold down the sides of the bag and scoop the cooled ganache into the bag. Twist the top of the bag to seal and force the ganache towards the piping tip. Pipe decorative swirls on top of the cooled ganache layer. Return the cheesecake to the refrigerator until ready to serve. Cheesecake can be stored in the fridge for up to 3 days, loosely covered.