For the master stock, use a paring knife and remove any white pith from the orange peel as this will impart a bitter flavour to the stock. Roughly crush the rock sugar using a mortar and pestle. Add all ingredients to 5 litres of water in a pan, bring to the boil, simmer for 30 minutes and then strain.
Bring master stock to the boil in a pot that can easily hold the ribs (cut racks in half if you need to). Add ribs to master stock and simmer until tender but still holding to the bone, for about 1½ hours.
Put sugar and ½ cup of water in a small saucepan over a medium heat.
Once sugar has dissolved, resist the urge to stir and instead gently shake and swirl the pan. Simmer until the mixture changes colour to a dark golden brown, then add vinegar and soy sauce, and bring to the boil. The sauce should have a mild burnt-sugar flavour.
Remove ribs from stock and, if you have time, leave in the fridge overnight to set. Separate ribs with a sharp knife.
Preheat a clean, well-oiled barbecue to medium. Cook ribs to heat through, about a minute each side – they will brown very quickly.
Arrange ribs on a serving plate, pour warm sauce over the top and garnish with chilli and coriander.