1: Put into a mixer bowl: 200 g butter cut into small pieces, 135 g icing sugar and 2 g fine (or table) salt.
Mix together on slow speed until evenly mixed (pastry cooks say "to cream", as the consistency becomes like a cream).
2: Add 50 g egg and mix again.
3: Finish by adding 300 g flour and 25 g powdered cocoa all at once, then knead for a few seconds until evenly mixed.
Stop the mixer as soon as the dough is ready; it is important not to over-knead it.
4: Weigh out the dough into pieces of 10-12 ounces (250-300 grams). Shape into balls, then flatten into "cakes" and wrap in plastic film.
Refrigerate until needed or freeze for later.
You can use this as the base for simple chocolate biscuits: make them more indulgent by adding chocolate chips to the dough at the end of kneading.