Wash the Urad Dal till water runs clear. Soak overnight in 4-5 cups of water – overnight would be perfect.
Next day drain the dal,and add the dal about quarter cup at a time. Avoid adding water while grinding, but if the mixer/ grinder jams, add as little water as possible. Remember to wipe off and mix back the dal that sticks to the walls. Stop once you get a smooth thick batter.Take out the batter into a container.Add salt into the batter.Keep aside.
Heat oil in a kadhai for deep frying the vadas. Check the oil drop some of the batter and if it quickly rises to top with slight bubbles its ready.
Now time to fry,Wet your hand/spoon, take golf ball size batter and gently slide it in the hot oil. Don’t fry more than 4-5 at a time.Turn them around a couple of times so that they are evenly golden on all sides. Once done, take them out with a slotted spoon and put then into the Luke warm water and after 1-2 minutes take out the vadas n drain the water as much as possible by pressing with help of two palms.Set all the vadas like this in a big flat vessel.
In a bowl whisk the curd with 1/3 tsp of salt,1/2 cup of water and 1/2 tsp of sugar.Now pour half of the curd all over the vadas and keep aside for 1/2 an hour.
For serving:-Put some of the vadas in a serving bowl and some of the whisk curd over it.Sprinkle some cumin powder,pinch of black salt and chat masala and pour some tamarind chutney over it and serve.