Mix the flour, icing sugar, cocoa powder and salt together in a bowl. Add the butter and work it in with your fingertips.
Stir in the eggs until the mixture comes together in clumps. Knead briefly until smooth then wrap in cling film and chill in the fridge for an hour.
Meanwhile, preheat the oven to 200°C/400°F/Gas Mark 6. Mix the mincemeat, marmalade and orange zest in a bowl and grease a 12 hole tart or bun tin.
Remove the pastry from the fridge and roll out onto a floured surface to a thickness of 5mm. Using a fluted 7cm cutter, stamp out 12 discs from the pastry and use them to line the prepared bun tin. Spoon a little mincemeat mixture into each pie case and brush the edges with a little milk.
Using a smaller 6cm 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 out star shapes to decorate the top. Brush the top of each mince pie with beaten egg and place the stars on top, then brush again with egg.
Bake in the oven for 30 to 35 minutes, allow to cool, then dust with icing sugar before serving.
