For the dough, put the milk in a heatproof jug and heat it in a microwave until boiling. Add 150ml/5½fl oz cold water to cool it down.
In the bowl of a freestanding mixer with a dough hook attachment, add the salt, sugar, egg, orange zest, lemon zest, flour and yeast. (Make sure the yeast and salt do not touch.)
Add approximately half the milk mixture and, using the dough hook attachment, mix slowly to bring the mixture together. Keep adding the milk mixture until all the flour is incorporated (you may not need all the milk). Increase the mixer speed to medium and mix for three minutes.
Slowly add the butter until incorporated and mix for a further six minutes. The dough should be smooth and elastic.
Grease a large bowl with rapeseed oil and add the dough. Cover with a shower cap or greased cling film. Prove the dough in a warm place (ideally 27C/80F) for 50 minutes, or until the dough has doubled in size.
When the dough has proved, tip it out onto a lightly floured work surface. Knock back the dough and fold over on itself a couple of times.
For the filling, gently knead in the cherries and chocolate chips until evenly distributed. Divide the dough into 80g/2¾oz portions and roll into balls.
Flatten the dough balls slightly and place half a sugar cube in the centre of each ball. Soak each sugar cube with some cherry brandy, using a pipette if available. Pull the dough over the sugar cubes, to seal the cube in the centre. Roll into neat balls.
Line two large baking trays with non-stick baking parchment. Place the dough balls on the baking tray leaving a 1cm/½in gap between the balls to allow for expansion. Spray with a fine mist of water (if you don’t have a spray bottle, wet your hands and flick droplets of water onto the buns). Prove for another 25 minutes in a warm place (ideally 27C/80F).
Preheat the oven to 180C/350F/Gas 4 and place a rack on the third shelf (just below the centre).
For the decoration, after proving, gently brush the buns all over with egg wash. Place a glace cherry on top of each bun and decorate with nibbed sugar. Using a small sieve, dust the buns with ground cinnamon.
Bake for around 25–30 minutes, or until well risen and golden-brown. Remove from the oven and place on a wire rack to cool.
Meanwhile, melt the dark chocolate in a heatproof bowl suspended over a pan of gently simmering water (do not allow the bowl to touch the water). Spoon the chocolate into a piping bag fitted with a fine writing nozzle.
When cooled, drizzle the melted chocolate over the buns to decorate. Serve once the chocolate is set.