Melt the black chocolate at 50°C.
Soften the butter in a microwave oven in steps of 30 seconds maximum to obtain a pomade.
Pour the softened butter in the bowl of a blender and add the icing sugar and the cocoa powder.
Raise with a blade on speed 2 during 5 minutes.
Incorporate the chocolate in three goes to avoid melting the butter and leave to turn on speed 1.
Meanwhile, raise the egg whites on speed 2 to a “beak shape” (the whites must be foamy without being too firm).
“Tighten” them with the sugar in two goes, and finish tightening on speed 3 for 20 seconds.
Mix the yolks and the whole eggs and add to the first chocolate based mix with a blade on speed 1.
Finish by delicately incorporating part of the egg whites by hand to avoid having the mix wither and pour back into the whites before delicately finishing the mix by hand.
On a baking plate covered with baking paper, lay a rolled-out pastry 35 x 35cm.
Bake in the oven at 190°C during 15 minutes and put away on a baking grate as soon as out of the oven.