Set your oven to 150°C/300°F and adjust a baking rack onto the middle shelf. Grease a 9-inch springform cake pan, then wrap it in a single large sheet of heavy-duty foil, covering the bottom and sides completely to prevent leaks.
Crush the cookies into fine crumbs using a food processor, then mix them with melted butter until evenly combined. Press the mixture firmly into the bottom of a springform pan to create a compact base.
Bake the crust for 7 minutes. Allow the crust to cool while you prepare the filling.
Place the chopped dark chocolate in a microwave-safe bowl. Microwave at 50% power in 30-second intervals, stirring until smooth. Stop when mostly melted, the residual heat will melt the rest as you stir. Allow to cool before use.
In a large bowl, beat the ricotta cheese with sugar, orange zest, a pinch of sea salt, and optional sour cream until smooth and lump-free (don’t overmix).
Whisk in one egg at a time, followed by the melted chocolate.
Place a sieve/ strainer over the bowl. Add the flour and cocoa powder, and sift into the cheesecake batter. Whisk until just combined (do not overmix). Pour the batter over the cheesecake crust and smooth the top with a spoon or spatula.
Bake for about 60 minutes, until the edges are set but the center has a slight jiggle. The center should still be slightly wobbly when you turn off the oven.
Once baked, turn off the oven, open the oven door, and let the cheesecake cool inside for about an hour. Then, move it to the counter before refrigerating. Refrigerate the chocolate ricotta cheesecake for at least 4 hours, but overnight is best for the perfect texture and flavor.