For the slow-braised, milk-fed lamb shoulder, preheat the oven to 180C/350F/Gas 4.
With the tip of a sharp knife, cut small incisions in the lamb shoulders at approximately 5cm/2in intervals.
Mix together the red chilli powder, ginger paste, garlic paste, vinegar, yoghurt, fried onions, royal cumin and salt, then massage them over the lamb, rubbing and pressing the spices into the knife cuts. Set aside for 15 minutes.
Put the shoulders in an oiled roasting tin. Add the bay leaves, cinnamon and green cardamom, and then cover the tin with foil. Place in the oven and braise for 2½ hours, or until the meat is very tender and ready to fall of the bone. Remove from the oven and leave to cool. Remove the shoulders from the liquid and allow them to dry. Pass the liquid through a fine sieve and reserve to make the sauce.
Take the meat off the bone and cut into 5cm/2in cubes. Thread them on to 8-10 metal skewers and roast on a barbecue or under a very hot grill, basting frequently with the melted butter, until crisp and well browned. Finish with a drizzle of lemon juice, any leftover melted butter and the chat masala.
For the cumin potatoes, put an empty roasting tin in the oven (one big enough to take the potatoes in a single layer) and preheat the oven to 200C/400F/Gas 6.
Drop the chopped potatoes into a large pan and pour in enough water to barely cover them. Add salt, then wait for the water to boil. As soon as the water reaches a full rolling boil, lower the heat, and simmer the potatoes uncovered, reasonably vigorously, for two minutes.
Meanwhile, put your choice of fat into the hot roasting tin and heat it in the oven for a few minutes, so it’s really hot.
Drain the potatoes in a colander and shake the colander back and forth a few times to bruise and fluff up the outsides. Sprinkle with the cumin and chilli flakes, and give another shake or two so they are evenly and thinly coated. Carefully put the potatoes into the hot fat – they will sizzle as they go in – then turn and roll them around so they are coated all over. Spread them in a single layer making sure they have plenty of room.
Roast the potatoes for 15 minutes, then take them out of the oven and turn them over. Roast for another 15 minutes and turn them over again. Put them back in the oven for another 10-15 minutes, or however long it takes to get them really golden-brown and crisp. The spices will speckle the potatoes.
For the saffron sauce, bring the reserved cooking juices to the boil in a small pan and simmer until the volume of liquid has reduced and the sauce is thick enough to coat the back of a spoon. Season with salt and pepper if needed and stir in the cream, garam masala, saffron and fresh coriander.
To serve, pour the sauce over the meat skewers and sprinkle over edible gold leaves for decoration. Scatter the potatoes with more salt and chopped coriander and serve straight away.