Preheat the oven to 180 C / Gas 4. Line a 12-hole muffin tins with paper cases.
For the buns:
Beat together the caster sugar and butter until light and creamy. Add each egg separately and mix well. Sift in the flour and cocoa powder and mix well to incorporate. Spoon the mixture into the prepared paper cases, filling only 2/3 full to ensure a good rise.
Bake in the preheated oven for 20 to 25 minutes until risen and a skewer inserted into the centre comes out clean. Remove from the oven and allow to cool completely on a cooling rack.
For the icing:
Mix the icing sugar with water, until you have a thick but smooth consistency. Add 1 or 2 drops red food colouring and mix well.
Once completely cooled, ice the muffins with the icing. Add any edible Christmas decorations as desired and serve.