Warm the milk in a saucepan until steaming. Remove from the heat, then add the butter. Swirl to melt the butter and cool the milk a little. Mix the cocoa with 2 tbsp boiling water, then set aside to cool.
Mix 500g flour, the cinnamon, sugar, yeast and ½ tsp salt in a bowl. When the milk mixture is still warm, add it to the flour bowl along with the egg, and mix to form a sticky dough (use a tabletop mixer if you have one). Continue mixing, then kneading, until the dough is less sticky and feels springy – about 8-10 mins. Add the dried fruit and mix again until the fruit is evenly spread through the dough.
Put a clean bowl on your scales. Scrape the dough into this bowl to weigh the full quantity, then return half of it to the original bowl. Add the cocoa mixture, dark chocolate and orange zest to one bowl, and the white chocolate and cherries to the other. Knead each dough for a further minute or until well mixed. Return each dough to a cleaned, oiled bowl, cover with a tea towel or cling film and leave somewhere warm for 1-2 hrs (depending on your kitchen temperature) until the dough has roughly doubled in size.
Knead and fold each dough a little bit to knock out some air. Divide each dough into six balls (use the scales if you want your buns to be perfectly sized.) To shape the buns, take a piece of dough in your palm, pull a corner into the middle and press to seal. Continue doing this around the edge of the dough to create a tight bun shape when you flip the dough over. Shape all the buns, then arrange them on a lightly oiled tray in a chequerboard pattern, leaving about 1.5cm around each one to allow space for rising. Cover the tray loosely with oiled cling film and set aside for 30 mins-1 hr until almost doubled in size again. Heat oven to 200C/180C fan/gas 6.
Mix the remaining our with enough water to make a thick paste, about 80ml. Transfer the paste to a piping bag and snip a small opening. Uncover the buns and pipe a line along each row, then repeat in the other direction to make crosses. Bake for 25-30 mins until the buns are golden brown. Leave to cool.
Warm the jam in a saucepan and brush over the buns for a glossy finish, if you like. To serve, split the buns, toast them under the grill and enjoy with salted butter.