Pour 500ml of water in a saucepan on a medium heat, and bring to the boil. Stir in the Kashmiri chilli powder, turmeric, coriander, black pepper, fresh ginger, green chilli and kokum and simmer for 15 minutes.
Add the prawns, cover with a lid and simmer for about 5 minutes. Uncover the pan and stir in the garlic and stir well to combine.
Heat the oil in a small frying pan over a medium–high heat until visibly hot, then add the mustard seeds.
When they start to crackle (after about 30 seconds), add the cumin seeds and curry leaves.
Allow to infuse into the oil for about 30 seconds, then pour over the cooked prawns. Season with salt and serve hot.