Take the pastry out of the fridge & leave to warm up slightly at room temperature for 20 minutes.
In the meantime, make an egg wash by whisking together the egg yolk & 1 tsp of cold water.
Brush a 12 hole muffin tray with the melted butter & set aside.
Roll the pastry out on a lightly floured surface to a thickness of 3 mm then cut out 12 circles using a 9½ cm cutter & another 12 with the 7½ cutter.Knead together the pastry trimmings & re roll if necessary.The larger pastry circles are for the bases & the smaller ones are the lids. Gently press the larger circles of pastry into the muffin tray, making sure that the pastry reaches the top of each hole.
Spoon an equal amount of filling into each mince pie then brush a light layer of egg wash around the pastry's top edge.
Place a pastry lid onto the top of each pie then press down to seal. Lightly brush the lids with egg wash then place the tray in the fridge for 30 minutes.
Preheat an oven to 180°c/160°c fan (356°f/320°f).
Take the mince pies out of the fridge & brush on another light layer of egg wash.
Using a knife, cut a small cross in the middle of each lid then sprinkle over the demerara sugar.
Bake in the preheated oven for 25-30 minutes, until the pastry is crisp & golden brown.
Leave the mince pies to cool slightly in the tray then lift them out with a small palette knife & transfer to a wire cooling rack, to cool completely.