Mix the flour, icing sugar, cocoa powder and salt together in a bowl. Mix in the butter with your fingertips until the mixture resembles breadcrumbs.
Stir in the egg until the mixture comes together in clumps (you may need to a little cold water). Knead the dough briefly until smooth, wrap in clingfilm and leave to chill in the fridge for one hour.
Preheat the oven to 200C/400F/Gas 6.
Mix the mincemeat, clementine zest and juice together in a bowl.
Remove the pastry from the fridge, and roll out to a thickness of 0.5cm/¼in. Using a 7.5cm/3in fluted cutter, stamp out 12 discs from the pastry and use them to line a 12-hole bun tin. Fill each hole with two teaspoons of the mincemeat mixture and brush the edges with a little milk.
Using a 6cm/2½in fluted cutter, cut out 12 circles and use them to top the mince pies, pressing the edges together with your fingertips. Re-roll any remaining pastry and cut our snowflakes or star shapes to decorate the top. Brush the top of each mince pie with beaten egg and gently place the decorations on top, then brush again with egg.
Bake in the oven for 20-25 minutes, or until the pastry is cooked through.