To make the black buttercream, melt the butter and pour it into a bowl. Sieve in the cocoa powder and stir in the icing sugar a little at a time to make a glossy and spreadable. If the icing is too thick, gradually add the milk until it reaches the desired consistency. Add a little of the black food colouring powder a little at a time until you get the desired depth of colour (you sometimes have to whisk for a while to get a uniform colour). Pipe over the cupcakes, sprinkle with the edible black glitter and top with your chosen halloween sugar decorations.