Sift the flour and cocoa into a large bowl, or the bowl of a stand mixer. Stir in the sugar and salt.
Rub in the butter with your fingertips, or the mixer. As there is a large proportion of butter to flour you will form a dough pretty quickly after getting to the breadcrumb stage.
Briefly knead the dough to make it nice and smooth.
Between two sheets of parchment, roll out the dough to 3mm thick and then place in the fridge to rest for 30 minutes
Preheat the oven to 160 degrees Centigrade
Stamp out shapes from the chilled dough and place, spaced apart onto a lined baking sheet.
Bake for 12-15 minutes
Sprinkle with caster sugar whilst hot and then move to a wire rack to cool