Cut the 2 types of chocolate into small pieces and pour them into a container suitable for a bain-marie.
Place it on a pan with water and put it on the stove over medium heat.
Melt the chocolate and add the sifted cocoa.
Remove from the heat and stir for a few minutes.
When it reaches the temperature of 50°C (122°F), start tempering the chocolate, it means pouring the chocolate on a metal or marble surface and with a steel spatula bringing the temperature back to about 32°C (89.6°F).
If you do not have this surface, work the chocolate very quickly with a spatula inside a steel bowl so that the temperatures drop to 32°C (89.6°F).
In the meantime, heat (do not boil) the whipping cream.
Turn off the heat and pour it into the chocolate mixture; mix well and pour the compost into a baking sheet (9 inches)lined with parchment paper and make sure the paper is as tall as the border.
Refrigerate for at least 24 hours.
Remove it from the fridge and from the pan by pulling at the parchment paper.
Put it on a serving dish and sprinkle it with bitter cocoa.