Wash and soak the bengal gram for 1 hour and grind it coarsely.
In a small sauce pan add prawns, 1 tsp ginger and garlic paste, 1 tsp red chilly powder, 1/4 tsp turmeric powder, 1/4 tsp salt and few tbsps water. Cook the prawns for 3 mins, remove from heat and set aside to cool. When prawns reach room temprature chop the prawns coarsely.
In a medium bowl add coarsely ground bengal gram, finely sliced onions, chopped curry leaves, coarsely chopped prawns, ground masala paste, turmeric powder, baking soda, required salt and 2tsp of oil.
Mix well so the bengal gram, prawn and onion mixture is well blended.
Heat enough oil in a pan to deep fry. When the oil is hot, divide the prawn vada mixture into equal sized balls. Flatten each ball between the palm and slide it into the hot oil, you can add maximum of five vadas at a time, depending on the size of the pan. Cook the vadas on medium heat until golden brown in color( Prawn vadai has to be crisp and crunchy outside and soft and well cooked inside). Once the vadas are done drain from the oil and transfer to paper towel. Serve hot.