1. Rub lemon juice, ginger-garlic paste, and salt on the prawns and keep aside for half an hour.
2. Roast the sesame seeds and crush to a powder.
3. Beat the yoghurt in a bowl and add the remaining ingredients (except chaat masala).
4. Rub this mixture on the prawns and keep in a cool place for 1 hour.
5. Preheat the oven to 150°C (300°F).
6. Skewer the prawns and roast till light golden in colour.
7. Apply the sesame seed powder over the prawns and roast again for 2 minutes.
8. Sprinkle with chaat masala and lemon juice.