2: Knead on low speed for 3 or 4 minutes until evenly mixed.
3: Crush 50 g dark chocolate into small pieces and add to the mixture with 1 teaspoon powdered cocoa. Knead for a further 2 or 3 minutes.
4: Gather the dough together and form into a roll about 4 cm (2") diameter, then wrap in plastic film and refrigerate at least one hour or overnight.
5: Preheat oven to 180°C (360°F). Remove dough from fridge and cut slices 0.5 cm ( ¼") thick. You will find that as you slice, the roll of dough will deform, so turn it as you go, and if necessary reshape the slices.
7: Cook for about 20 minutes, then leave to cool on a wire rack.
Remarks
It is important not to overcook these cookies, otherwise they will become dry and less tempting. You can use commercially prepared chocolate chips rather than breaking up a chocolate bar.