Melt the chocolate in pieces in a saucepan placed in a bain-marie.Stir with a wooden spoon. Gradually add the butter cut into pieces. Remove the pan from the heat, add the egg yolks, then the sifted icing sugar and the vanilla sugar.
Leave to rest for about 2 hours in the refrigerator for the chocolate to harden.
Form small balls of cooled chocolate, using a small spoon then roll them quickly between your hands and roll them in the cocoa powder.
You can then keep them for several days in an airtight box at room temperature (in winter) or in a cool place.
