Sieve the flour and baking powder and then confectioners sugar into a bowl. If you omit this stage you may get sugar lumps in your dough.
375 g flour, 120 g powdered sugar, 2 teaspoons baking powder
Add in butter and salt. Knead with both hands until you achieve a smooth dough.
250 g butter, 1 pinch salt
Half the dough. Add the cocoa to one half and knead until it is mixed in well. Now divide the white and the brown dough into 8 equal-sized balls each.
1 tablespoon cocoa powder
Take one of each and knead together until you achieve a nice marble pattern. Repeat until you have 8 equally sized marbled balls. Now combine them into one big roll which has a (Ø 3,5 cm/ 1 inch)Coat the roll in granulated sugar before wrapping it in cling film and leaving it to rest in the fridge for around 1 hour. The butter will firm up again, and it will make it easier to cut later on.
2 tablespoon granulated sugar
Cut the cookies
In the meantime preheat the oven to 160°C/ 320°F circulated heat, 180°C/ 356°F top to bottom heat.
Slice the roll into little biscuits, just less than 0,5 cm thick, and place onto a tray lined with on some baking paper
Bake for around 10-12 minutes. They will be a little soft when you take them out of the oven but they will harden once cooled.
Notes
They should keep in an airtight container up to three weeks.