Place the bread flour in a large bowl and add the cocoa powder, mixed spice, salt and sugar to one side and the yeast to the other (yeast doesn't like to come into direct contact with salt). Mix it all together.
Place the butter, milk and orange zest in a pan and heat until the butter has melted. Set aside until it has cooled to lukewarm (it shouldn't be so hot that you can't comfortably hold your finger in it). Pour the milk mix into the flour along with the egg and stir until it forms a rough dough.
Turn out onto an un-floured surface and knead for about 10 minutes until smooth and elastic. The dough should be sticky at first but become smooth as you knead it. Alternatively you can use a stand mixer fitted with a dough hook to knead it for about 6-7 minutes.
Place the dough in a lightly oiled bowl and cover with clingfilm. Set aside to rise in a warm place until doubled in size, about 1-2 hours.
Flatten the risen dough out into a rectangle and scatter over the dried fruit and chocolate chips; knead until they are well distributed. Divide the dough into 15 equal pieces and shape each one into a ball. Place them slightly spaced apart on a baking tray lined with baking parchment. Loosley cover with oiled clingfilm and set aside to rise until puffy, about 1 hour.
Meanwhile, preheat the oven to 200°C/400°F/gas mark 6. To make the crosses, place the flour in a bowl and slowly whisk in enough water to form a thick, pipe-able consistency; your looking for something with a similar consistency to wallpaper paste. Place the paste in a disposable piping bag and snip off the very tip (or put it in a piping bag fitted with a small round nozzle).
Pipe a cross on the top of each risen bun then bake them for about 20 minutes. They should be well risen and browned; if you have a probe thermometer then the internal temperature should reach at least 94°C/201°F.
While the buns are baking prepare the glaze. Place the orange juice and sugar in a small pan; heat while stirring until the sugar has dissolved then bring up to a simmer and allow to simmer for about 5 minutes until syrupy.
Once the buns are baked, remove them from the oven and brush the tops liberally with the glaze. Allow to cool then serve them split, toasted and buttered.