To make the chocolate shortcrust pastry, sift the flour in a large mixing bowl, add the cold butter cut into cubes, and use your fingertips to rub them together until you get a mixture that resembles breadcrumbs.
Add the sugar, and mix to combine them.
Sift in the cocoa powder, and mix again.
Beat the egg well, then add it to the bowl together with the milk.
Use your hands to knead the mixture into a dough that is elastic and does not stick to the hands.
Preheat the oven to 180 degrees Celsius (350 Fahrenheit).
Transfer the dough to the work surface, and use a rolling pin to roll the dough, then use a 10 cm round cookie cutter to cut out 12 rounds, reworking the dough as needed.
Arrange the rounds into each cavity of a 12-hole mufin tin, and fill with mincemeat.
From the remaning dough, cut out stars to fit on top of each mince pie.
Bake in the preheated oven at 180 degrees Celsius (350 Fahrenheit) for 25 minutes.
Remove the pies from the oven, and allow them to cool down in the tin for 15 minutes, then use a sharp knife to run around each pie to pop them out of the muffin tin.
Leave the pies to cool down completely, then dust with icing sugar.