Heat your oven to 180°C Fan/200°C/390°F/Gas mark 6.
Firstly, take your puff pastry out of your refrigerator and leave it to warm up a little, so that you can roll it out. 10 minutes should be enough, but it depends on your climate.
Lightly grease your baking sheet with some butter and then cut a piece of greaseproof (parchment) paper large enough to cover the base of your baking tin and place it onto the tin, pressing down so that the buttered base affixes it.
Take all of your mincemeat ingredients and mix thoroughly together.
Flour your work surface and then roll our your puff pastry to a thickness of 3mm.
Using your largest cutter, cut out 24 circles.
Place 12 of your cut circles onto your baking sheet, leaving a gap of 1cm between each one, so the pastry has room to expand.
Take a teaspoon of your mincemeat and pop it into the centre of your pastry circle.
Wet around the outside of the pastry circles, so that the pastry lid has something to stick to and place your 12 remaining lids on top.
Press the lids gently down onto the base circle so that they fuse together a little.
To secure the lids fully, take your smaller cutter, turn it around so you are not using the cutting edge, dip it in some flour and then place it over the lid and press down, so that it pushes the lid into the base a little and affixes it. Don't press too hard though or you will go through the base circle completely.
Take your egg yolk and liberally brush over your puff pastry mince pies using your pastry brush.
Sprinkle over some demerera sugar, this will give a lovely crunch to the top.
Then take your knife and cut a slit in the top of the mince pie, this will release any trapped steam and help the mince pie to cook throughout. Waggle the knife from side to side a little so as to open up the hole slightly.
Bake in the centre of your oven for 20 minutes, the mince pies should have risen and be golden brown on top when ready.
Leave to cool in the tin for 5 minutes then remove to your cooling rack to continue to cool.