Cream the butter (softened) with the vanilla and sugar until pale.
Preheat the oven to 180C. Grease an oven tray or line with baking paper.
Sift the flour with the cocoa. Fold the cornflakes into the butter mixture, then shape into rounds on a baking sheet. Bake for 20 minutes then let cool.
Sift the icing sugar with the cocoa. Add the butter and vanilla then stir in enough boiling water to make a smooth icing and ice the biscuits.
Chef's tip The quality of the cocoa can make a huge difference to the outcome of both the flavour of the biscuits and texture of the icing on the afghans. Take the time to find a pure cocoa product without additives. A small jar of the best quality cocoa you can afford may be pricey, but it's worth it to get such a delicious result.