1. Peel the prawns carefully, leaving the tails attached.
2. Using a small sharp knife, make a slit along the back of each prawn and remove the dark vein. Rinse under cold running water, drain and pat dry.
3. Put the ginger, garlic, chilli powder, turmeric, mustard seeds and cardamom seeds in a bowl. Add the prawns and toss to coat completely with spice mixture.
4. Heat a wok until hot. Add the ghee or butter and swirl it around until foaming.
5. Add the marinated prawns and stir-fry for 1-1½ minutes until they are just turning pink.
6. Stir in the coconut milk and simmer for 3-4 minutes until the prawns are just cooked through. Season to taste with salt and pepper. Sprinkle over the coriander and serve at once with naan bread.