Remove the shells from the prawns. Rub in some coarse salt and rinse well, a few times. Squeeze out all the excess water or use paper towels to pat them as dry as possible. Marinate the prawns with the chilli powder, garlic, cumin and salt. Refrigerate the prawns for 30 minutes
Heat 2 tablespoon oil and fry the prawns, preferrably in 2 batches, about 20 seconds on each side. Do not over fry them. It's ok if undercooked, it will cook further in the sauce
In the same pot add another 2 tablespoon oil. Add the cinnamon stick, bay leaf, mustard seeds and fennel seeds. Fry until fragrant and the seeds start to splutter. Add the onion, curry leaf and chillies and fry until the onion is golden brown
Add the ginger garlic paste and fry for a minute. Add the blended tomatoes and cook for 5 minutes. Add the masala, turmeric, garam masala, cumin, coriander and fennel. Add a little water. Cook for a further 2 minutes
Add the potatoes and mix well. Season with salt. Cook for 1 minute. Mix the tamarind into the hot water and strain into the dish. Depending on altitude you may require less or more than a cup of water. Allow the curry to simmer until the potatoes are tender. Add the cooked prawns once the potatoes are completely cooked. Bring to a quick boil for 2 minutes and turn the heat off.
Garnish with coriander and serve hot