Line two 12-hole muffin tines with paper cases. Sift the cocoa powder into a bowl, pour in the boiling water, and mix into a thick paste. Add the remaining cake ingredients and mix with an electric hand whisk (or beat well with a wooden spoon).
Divide the mixture equally among the 24 paper cases. Bake in a preheated oven at 200˚C for about 10 minutes until well risen and springy to the touch. Cook in the cases on a wire rack.
Make the icing. Melt the butter, then pour it into a bowl. Sift in the cocoa powder and stir to mix. Stir in the milk and then sift in the icing sugar a little at a time to make a glossy, spreadable icing. Spread the icing over the cakes and leave to set before serving.