Preheat oven to 180°C (160 fan) | 350°F | gas 4.
In a large bowl, stir together the icing sugar, cocoa, flour and salt until well blended. Cut in the butter until the lumps are no larger than peas.
Add the eggs and vanilla then mix until a stiff dough forms (this may take a couple of minutes - if the dough is too sticky, leave to chill in the fridge for a few minutes).
Shape the dough into a 5 cm? log then cut the log into biscuits using a sharp knife. Transfer the cookies to an ungreased baking tray, spacing them about 5 cm apart.
Bake for 10 minutes or until cooked through. Remove from the oven and immediately dust with the caster sugar. Cool for 5 minutes on the tray before moving to a wire rack to cool completely.