Lightly toast the whole spices for the masala in a dry pan for a minute. Cool and powder.
Drain the prawns of all the water, pat dry with kitchen towel and toss in 1/2 tsp each of salt and turmeric. Kepp aside for 10 minutes.
Place the prawns in a wide pan and cook on low heat. The prawns will start emitting water. Cook them gently till all the liquid has dried.
Heat a tbsp of gingelly oil in another pan and lightly fry the prawns for 3-4 minutes; they will change colour. Remove from the oil and set aside.
Add the remaining oil to the pan and toss in the ginger, garlic and curry leaves. Fry till golden (be careful not to burn the aromatics).
Keep the flame low and mix in the remaining ground masala, salt and vinegar. Bring to a gentle boil, add the prawns, stir well for 15-20 seconds and turn off the heat.
Allow the pickle to cool before spooning into a sterilised jar reserving some of the oil. Pour the oil on top to cover the pickle, close the jar tightly and refrigerate for 5-6 days before eating.