Dry roast vermicelli till light golden. Set aside to cool. Whisk sour yogurt till smooth. Add it to the cooled vermicelli along with half a cup of water. Mix, cover and let it rest for about fifteen minutes. Add green chillies, coriander leaves, curry leaves, salt, soda bicarbonate to the vermicelli mixture and mix well. Heat oil in a small pan and add mustard seeds. When they crackle, add cashewnut pieces and sauté till golden. Pour this into the vermicelli mixture and mix. The consistency of the batter should be thick (but pouring) like idli batter. Grease the idli mould. Heat sufficient water in a cooker. Pour the vermicelli batter into the moulds till half full. Place the mould in the cooker, cover and steam for fifteen minutes. Insert a needle to see if the idlis are cooked through. Serve hot with coconut chutney or with molagapodi.