Line two baking trays with baking parchment. Put the shortbread in a food bag and crush to fine crumbs, then tip into a large bowl. Add the mixed spice, mincemeat, orange zest, cream and a pinch of salt, and mix until combined. Melt the milk chocolate in a heatproof bowl set over a pan of simmering water or in the microwave in short bursts, then stir this into the mixture. Chill for 45-50 mins until firm enough to roll into balls.
Roll a heaped teaspoon of the mixture into a ball and put on the lined tray. If the mixture is a little sticky, dampen your hands slightly. Repeat with the rest of the mixture and chill for 45 mins-1 hr more until set.
Melt the dark chocolate as before, then leave to cool slightly and pour into a small, deep bowl. Dip each truffle into it using a fork and shake off the excess. Put on the second tray to set. When set, decorate with edible gold lustre, if using. Will keep in a cool place or the fridge for a week.