Mix the custard with the egg yolk and corn-starch ( if used) and set aside. The custard should be very thick.
Preheat the oven to 190°C.
Unroll the pastry onto a flat surface.
Using a mug or cookie cutter, which is about a centimetre wider than the holes in a shallow baking tin, cut out rounds. Press them lightly into the tin holes.
Add a spoonful of custard, then top with a spoonful of mincemeat.
Optional: If you want stars on the top, roll out any leftover pastry and, using a small star cutter, stamp out star shapes. Put these on top of the pies (the tip of the stars should meet the edge of the pie)
Put the tin into the preheated oven and bake the mince pies for about 15- 20 minutes, until they’re golden brown.
Transfer them onto a wire rack and let them cool.
Dust the mince pies with icing sugar just before serving.