Peel (leaving the tail on), de-vein and wash the prawns; leave to drain and pat dry with kitchen towel.
For the first marinade:
Sprinkle juice of one lime and ½ a teaspoon of salt, mix well and leave aside.
For the gram flour roux:
Heat 2 tablespoons of rapeseed oil in a thick-bottomed pan, add the gram flour, stir constantly over a low flame until it turns a very light brown in colour; transfer to a plate to cool.
For the second marinade:
In a mixing bowl, add yogurt, paprika, chilli powder, turmeric powder, ajwain seeds, nigella seeds, ginger and garlic paste, salt, oil and the gram flour paste; mix well.
Now add in the prawns.
Leave refrigerated for about 20 minutes and bring to room temperature before grilling in a tandoor until cooked to required doneness. To cook in an oven, pre-heat to 200c / gas mark 6.
To grill, place the marinated prawns on a cooking rack (with a drip tray below) and grill in the oven for 4 minutes, turn the pieces and grill for 3-4 minutes or until done. Remove and set aside to cool.
Serve hot, garnish with chives, melted butter and chaat masala; accompanied with baby leaf salad.