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