In a large bowl, rub the flour into the butter using your fingertips. Make a well in the centre and add the egg yolk and 1 tablespoon of cold water. Using a knife, work into a dough, adding a drop more water if necessary. Knead gently until smooth. Wrap in clingfilm and chill in the fridge for 15 minutes.
Preheat the oven to 200°C, fan 180°C, gas 6.
On a floured surface, roll out the pastry to a thickness of 5mm. Using a 7cm round cutter, stamp out 12 rounds. Line a 12-hole tart tin with the pastry rounds and spoon a generous teaspoon of mincemeat into each case. Roll out the pastry trimmings, cut out small stars and trees, and place on top of the mince pies.
Bake in the oven for 10 minutes. Reduce the oven temperature to 180°C, fan 160°C, gas 4 and bake for a further 8-10 minutes, until golden. Cool on a wire rack, then dust with icing sugar.