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Wrap the butter in parchment and put it in the freezer for 1 hr before using. Tip the flour into a large bowl with ¼ tsp salt and mix together. Take the cold butter and coarsely grate it into the flour, dipping the end in as you go to stop it clumping. Mix in the orange zest and caster sugar, then swiftly add the orange juice and 2-3 tbsp cold water. Bring together with your hands then wrap and chill for at least 1 hr.
For the filling, heat the custard in a pan over a low heat. Mix the rum with the cornflour, then whisk it into the custard. Bring to a simmer and cook, stirring continuously, for 3 mins. Strain to remove any lumps. Set aside to cool, then chill for 1 hr – it should be thick once cooled.
Heat the oven to 200C/180C fan/gas 6. Butter a 12-hole fairy cake tin. Mix the mincemeat with the apple in a bowl. Roll out the pastry on a lightly floured surface to the thickness of a £1 coin and cut out 24 rounds with an 8cm cutter. Re-roll any scraps as you go to get the full amount. Place 12 of the rounds in the holes. Put a spoonful of the mincemeat into each one, followed by a heaped teaspoon of custard and a little more mincemeat. Top with the remaining pastry rounds, seal the edges with a fork and cut a tiny cross on the top of each one with a sharp knife. Brush with egg and sprinkle with the demerara sugar. Bake for 25-30 mins or until golden and crispy. Leave to rest for 10 mins before serving warm.