Heat oven to 200C/180C fan/gas 6. Put a baking sheet in the oven on the middle shelf.
Cut the butter into small cubes. Sift the flour into a large mixing bowl and add a pinch of salt.
Tip the butter into the flour and use your fingers to rub the two together until it resembles breadcrumbs.
Stir in 2 tbsp of the sugar. Scoop a heaped half cup (85g) of the crumbs into a smaller bowl and set aside.
Have two small bowls ready. Crack the egg on the side of one bowl, then carefully pull the shell apart in two halves, letting the yolk nestle into one side. Let the white drain away from the yolk into one bowl and put the yolk into the other bowl.
Beat the egg yolk with a fork, then add it to the bigger bowl of butter and flour mix. Use a cutlery knife to stir the yolk into the crumbs.
Put your hand in the bowl and squeeze the crumbs together to make a ball. Knead the pastry a few times until smooth.
Sprinkle some flour over your work surface. Sprinkle a little more over the pastry and roll it out to the thickness of two £1 coins.
Stamp the pastry with an 8cm cutter, re-rolling the trimmings to make 12 rounds.
Gently push the rounds of pastry into the bun tin. Spoon 1 heaped tsp of the custard into each pastry case, followed by the mincemeat on top.
Add the final 1 tbsp sugar to the smaller bowl of crumb mix. Pinch it together to make bigger clumps and mound it on top of the mincemeat.
Slide the bun tin on top of the baking sheet and bake for 15 mins until golden brown and the custard and mincemeat are starting to bubble.
Cool in the tin for 15 mins, then carefully lift the pies onto a rack and leave to cool. Will keep for 2 days in an airtight tin.
Make them lastAs they have a custard layer, these pies won’t last very long in an airtight tin. If you prefer, leave out the custard and use 2 tsp mincemeat per pastry case before adding the crumble top. You can eat them with custard instead!