Soak the two dals for 7-8 hours or overnight. Grind it to a smooth paste.
Heat oil for deep frying in a kadahi and add spoonfuls of dal batter and fry until the vadas are golden brown.
Drop the hot vadas in a bowl of cold water and leave for about 3-4 minutes.
Take each vada out from water, squeeze the water out and set aside. Method for making dahi
Blend the yogurt with little water until it is smooth.
Add sugar, salt, green chillies and ginger.
Make a tarka (seasoning) of mustard seeds, asafoetida and curry leaves and add to yogurt.
Keep the yogurt in refrigerator until it is chilled.
Serving the vadas: Arrange the squeezed vadas in a deep dish. Pour the yogurt on the vadas such that all the vadas are covered well by the yogurt. Sprinkle red chilli powder, roasted jeera powder and black salt (sanchal).