Sift the flour and cocoa into a bowl and rub in the butter until it has the appearance of chocolaty breadcrumbs.
Chuck in the caster sugar, vanilla and the egg and knead together until the mixture becomes a thick gooey lump.
Sprinkle a bit of flour over your work surface and roll the lump out until it's around 5mm thick (though you can go thinner or thicker if you like).
Cut into rounds using a pastry cutter (or the top of a coffee jar if you like!) and place onto a greased baking tray. The biscuits can go as close as you like. They will spread out but will be dead easy to separate once cooked.
Bake in a preheated moderate oven (180 C / Gas 4) for about 8 minutes.
While the biscuits are cooking, mix up a little icing sugar. Add water VERY slowly! You only need a tiny bit to make a thick, gloopy icing sugar mix.
When cooked, leave the biscuits for a couple of minutes to harden, then transfer them to a rack to cool. Dollop on 1/2 a teaspoon of icing sugar onto each.
Put the kettle on, have a cup of tea and enjoy a biscuit or two before everyone else nabs them!