Line a 12-section bun tray with red foil cake cases. Put the cocoa powder into a bowl and mix to a smooth paste with the boiling water.
Put the butter, eggs, caster sugar and flour into a bowl and beat until smooth. Stir in the cocoa paste, then divide the mixture among the paper cake cases. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 13–15 minutes until they are well risen and spring back when pressed with a fingertip. Leave to cool.
Melt the chocolate in a heatproof bowl over a pan of gently simmering water. Spoon most of the melted chocolate (reserve a little and keep warm in the bowl) into a paper piping bag, snip off the tip and pipe lines of chocolate about 6 cm (2½ inches) long on a baking tray lined with nonstick baking paper. Pipe on branches for antlers. Make enough for 2 per cake, with extras in case of breakages.
Mix the icing cocoa powder with the boiling water. Add the butter, then gradually beat in the icing sugar until smooth. Spread over the tops of the cupcakes.
Add to each a cherry half for a nose and 2 chocolate drops for eyes, piping on the remaining melted chocolate to make eyeballs. Peel the set antlers off the baking paper and stick at angles into the cupcakes.