1. Reserve 2 tablespoons lemon juice and mix the rest with the ginger and garlic pastes.
2. Add salt, chick-pea flour, carom seeds, yogurt, yellow chilli powder, garam masala and turmeric.
3. Marinade the prawns for 2 hours.
4. Arrange on skewers and cook in a tandoor till half done, about 15 minutes.
5. Remove and set aside for 10 minutes.
6. Cook again in the tandoor for 2 minutes.
7. Baste with butter and replace in the tandoor for approximately 3 more minutes.
8. Remove from skewers and arrange on a platter.
9. Sprinkle with chaat masala and the reserved lemon juice.
10. Serve with a green salad.