Lightly butter a 6-inch / 15cm springform pan or equivalent. I typically use small loaf pans which are less common, but many small pans will work here (see main entry). Line with parchment paper and set aside.
Pour the chocolate into the cream, and very slowly and steadily stir until everything comes together smoothly. Make sure the components aren't at all separate. Pour the mixture into the prepared pan, cover with plastic wrap, and refrigerate until chilled throughout, a few hours, or overnight.
When ready to serve, remove from the pan, let set at room temperature for ten minutes or so, dust with a bit of cocoa powder, sprinkle with any other toppings you like, and slice. Alternately, you can slice and serve from the pan.