Take prawns in a bowl, add turmeric powder, salt and mix well. Set aside to marinate for 30 minutes.
Add all ingredients for ground masala except for vinegar to a mixer. Add ¼ cup of vinegar and start grinding the spices. Add 2-3 tablespoons of vinegar at a time and grind to a smooth paste. This method of grinding ensures the smooth grinding of paste.
Heat sesame oil in a kadai or pan, add the marinated prawns and fry them on low heat for 5-6 minutes till the moisture evaporates and prawns get roasted on the edges to a golden brown. Remove from oil and set aside on a plate.
Heat the same kadai, add garlic, ginger and green chillies and saute for 1-2 minutes till they are fragrant. Add 2 sprigs of curry leaves and saute for a few seconds. Add the ground paste, salt and roast on low for 10-12 minutes till the masala gets roasted and oil separates from the side of the pan. While frying, if the masala sticks to the sides of the pan, scrape them out and add to the masala getting roasted. This will add more flavour to the pickle.
Add roasted prawns, mix well and cook for 2-3 minutes. Add hot water, mix well, cook for 2-3 minutes, add the remaining 1 sprig of curry leaves and take off the heat.
Cool completely, store in an airtight glass container and refrigerate.