Photograph: Martin Poole
For the pastry, mix the plain flour, unsalted butter and icing sugar in an electric mixer, until it resembles breadcrumbs. Briefly mix in the lightly beaten egg. When the pastry comes together, tip on to the surface and bring it together in a ball. Press the ball into a disc, wrap in clingfilm and chill for 30 minutes.
Meanwhile, beat the egg white until lightly frothy and spread 1 tbsp caster sugar out in a saucer. Dip the cranberries, one by one, in the egg white, then roll in the sugar to 'frost' them. Leave to dry.
Roll out the pastry on a floured surface. Use a fluted 9cm cookie cutter to stamp out rounds to line the holes of 2 x 12-hole muffin tins. Chill for 15 minutes. Preheat the oven to 180°C, fan 160°C, gas 4.
For the filling, melt the dark chocolate. Mix the butter and light muscovado sugar in an electric mixer to combine. Gradually add the lightly beaten eggs, still mixing. Tip in the sifted plain flour and the slightly cooled melted chocolate. Mix until combined. Fold in 250g mincemeat.
Put a spoonful of the filling into each pastry case. Bake for 20-25 minutes. Leave the pies to cool in the tins for 5 minutes, then transfer to a wire rack.
Whip the double cream with orange zest to top the cold pies; add the frosted cranberries.
Get ahead: the pies can be made a few days ahead and kept in an airtight container, or they can be frozen (without the topping). Finish with the whipped cream and cranberries just before serving.