Put the hung yoghurt in a large bowl and beat with a fork, then add the chilli powder, ginger, ginger paste and lemon juice. Season with salt and mix well.
Heat the oil in a pan to 180°C / 350”F, or until a cube of bread browns in 30 seconds. Add the turmeric and pour this over the yoghurt mixture. Cover the bowl with a tight lid for 2 minutes, then mix well.
Add the prawns to the yoghurt mixture and turn until they are evenly coated. Cover and allow to stand for 30 minutes.
Prepare a tandoor or charcoal grill for moderate heat, or alternatively preheat the grill (broiler) to medium.
Thread the prawns through several metal skewers and cover them with the remaining marinade.
Cook in a moderately hot tandoor, over a charcoal grill, or under the hot grill for 8-10 minutes, or until the prawns are cooked.
Remove the skewers from the heat and hang them up over a tray for about 5 minutes to allow any excess moisture to drain out completely.
Put the skewers in a dish, baste with melted butter, and roast again in the tandoor or under the grill for about 2 minutes.