Before you start, put the chocolate in the freezer for 10-15 minutes and then grate it.
Combine the sifted flour, cocoa, baking powder and salt in a suitable bowl. Separately, beat the butter and sugar gently with a mixer on low heat, until mixed properly. Add the egg yolk and beat again gently without beating to a smooth cream.
Sprinkle the chocolate on the kitchen counter and pour the mixture from the bowl on top. Knead with your hands until the chocolate is absorbed and you get a buttery dough.
When it has a uniform consistency, stop kneading. Divide the dough into 3 parts and wrap each in cling film. Leave in the refrigerator for 1 hour. Roll out the balls into a crust with a thickness of 1/4" (5 mm) and cut out small round biscuits with forms or small cups.
Work quickly to prevent the dough from softening, removing the balls one after another from the fridge.
Arrange the biscuits in an oven dish covered with baking paper and bake for about 12 minutes in a preheated 356°F (180 °C) oven.
Leave the chocolate biscuits to cool for a few minutes and then peel off the paper.