Prepare the shortcrust pastry and line the greased pie pan or a greased springform pan with it. Chill for at least 30 minutes.
Melt the butter with the chocolate and remove from the heat.
Beat eggs with sugar until creamy, stir in the chocolate mixture, cocoa and nuts and spread the mixture on the cold base.
Bake for 30 minutes at the same temperature.
Place the walnut halves on the cake. If you like, you can stick the nuts on with melted chocolate or Nutella.
If you are not into dark chocolate and prefer it sweeter, you can of course also use normal dark or even milk chocolate - it will of course be a lot sweeter. It would also be worth trying with white or nougat chocolate.