For the pastry, pour boiling water into a small saucepan about a third full and sit a small bowl on top. Add chocolate and allow to stand until chocolate melts, stirring occasionally.
Meanwhile, place the cocoa, flour, caster sugar and butter together in a food processor and pulse until mixture resembles fine breadcrumbs. In a small bowl, combine egg yolk, milk and vanilla, whisking until combined. Using a spatula, add to flour mix and pulse until combined. While the food processor is running, slowly add the melted chocolate until the mixture starts forming a ball. Remove dough onto a lightly floured surface and bring together. Flatten into a disc, cover with plastic wrap and refrigerate for 2 hours.
For the mince, place the apple, cranberries, currants, sultanas, peel, sugar, spice, zests, juice and sherry together in a saucepan and stir over a medium heat until the sugar has dissolved. Cook for 20 minutes or until liquid has been absorbed by the fruit. Set aside and allow to cool completely. Add the chopped chocolate, stirring until well combined.
Preheat oven to 180°C (160°C fan-forced). Lightly grease a 12-hole shallow muffin pan.
On a lightly floured kitchen work surface, roll pastry to a thickness of approximately 3mm. For the bases, cut out 12 pastry rounds using an 8cm cutter and for the tops, cut out 12 pastry rounds using a 7cm cutter. Line the prepared pan with the pastry bases and place a heaped tablespoon of cooled fruit mince into the centre of each tart.
Lightly whisk the remaining egg and brush over one side of the pastry top. Place on top of the fruit mince filling and press down the edges to seal the pies. Brush pie tops with egg glaze and sprinkle with coffee sugar. Using a skewer, create a small hole in the centre of each pie to allow steam to escape.
Place baking pan in preheated oven and bake for 25‒30 minutes or until pastry is cooked. Remove from oven and allow to stand for 5 minutes. Carefully remove each pie using a knife or spatula and place on a wire rack to cool.