1. Preheat the oven to 350F or gas mark 4. Line the muffin tins with paper cases.
2. Whisk the egg in a bowl. Add the milk, melted butter, sherry and the Christmas pudding.
3. Sift the flour, baking powder and mixed spice into another bowl. Add the caster sugar and mix well. Make a well in the centre.
4. Add the wet ingredients and mix together until a soft batter forms. Spoon the mixture into the paper cases and bake in the oven for 20-25 minutes. Allow to cool on a wire rack.
5. While they are cooking, make the sherry butter icing. Mix together the icing sugar and the butter. Add enough sherry, about 1 tbsp, to just soften the mixture and make it spreadable.
6. Once the muffins are cool, spread the icing over the top and serve.