Place prawns in a bowl. Add salt and turmeric powder. Mix and let them marinate for fifteen minutes.
Place three-fourth cup of coconut in a mixer jar. Add half cup of hot water and grind. Place a muslin cloth over a bowl and pour the coconut over it. Gather the edges of the cloth and squeeze tightly to extract thick coconut milk.
Slit green chillies. Add the remaining mustard oil to the same pan and heat. Add cumin seeds and mustard seeds and cover the pan and allow the seeds to splutter.
Add green chillies, prawns, coconut milk and cook on medium heat for half a minute.
Add chilli powder, cumin powder, sugar and ginger paste and mix. Cook till the prawns are done.
Garnish with remaining scraped coconut and serve hot.