Preheat the oven to Gas Mark 3, 170°C, fan 150°C. Line a 12-hole muffin tin with paper cases or grease the holes with butter. Melt the butter, muscovado sugar and golden syrup in a saucepan over a low heat and set aside to cool slightly.
Sift the flour into a bowl, then add the ground ginger, clementine zest, salt and bicarbonate of soda. Add the melted butter, sugar and syrup mixture, eggs, buttermilk and stem ginger. Mix thoroughly and pour into the cases or tin holes. The mix will reach quite high up the sides. Bake for about 25 minutes until golden and springy. Don't worry if they don't rise particularly high - the sponge is intended to be dense and sticky.
Cool in the tin - this makes the cupcakes more squidgy. (If you prefer a really strong, rich flavour, and can bear to wait, wrap the whole tin up tightly in cling film or foil and keep for 1-2 days before adding the icing. The sponge will get darker over time.) The cupcakes taste great straight from the oven, though, if you can't wait.
For the icing, beat the Philadelphia in a bowl until smooth. Sift the icing sugar over and gradually beat in, followed by the clementine zest and juice. Smooth a generous amount over the top of each cooled cupcake and decorate with extra zest and stem ginger.
Freezing and defrosting guidelines
Bake according to the recipe but stop before adding decoration or icing. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months.
Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving. If you wish to serve the baked goods warm, place them on a baking sheet for 10 minutes in a moderate oven.
If you wish to add icing or decoration, add them after the baked goods have fully defrosted and are at room temperature.