Wash the split (and hulled or skinned) urud dal and soak in 
sufficient water for about three hours.

At the end of three hours, drain water from the dal and put 
it in the food processor. Add water such that the dal is 
just immersed. Add salt to taste too. Grind into a semi 
soft paste. Do not make it too soft. While grinding the 
dal, add a little water as and when required. 

In a bowl, take the batter, add the baking powder and whisk
until the batter become light and airy. The consistency of 
the batter is such that it will stand up stiff, if a spoon 
is inserted in the batter and pulled out. Also, the batter 
turns whiter as you whisk it.

Meanwhile, mix the turmeric to the yogurt and whisk with an 
egg beater.

In a small skillet, heat oil and add mustard seeds. When 
they pop, add chopped green chilis, chopped coriander and 
curry leaves. Fry for a minute on medium and then remove 
from heat. First, add the ginger pieces. Then, add the 
yogurt and turmeric mixture and mix well. Keep aside.

Deep frying the vadas:
Heat oil in a deep skillet. When oil is hot, take slightly 
larger than walnut sized amount of batter and spread it on 
a wet plastic sheet in the shape of a disk (of half inch 
thickness and 2.5 inches diameter). Make a hole at the 
centre with your index finger.
Peel away the plastic sheet and at the same time, let slip 
the donut shaped batter into the hot oil. Cook to golden 
brown on both sides. In one batch, you may fry about three 
to four depending upon the width of the skillet.

When the vadas turn golden brown on both sides, remove from 
oil and dip in a container filled with cool water. Wait for 
two minutes and when you notice the vada sinking, remove 
from water with a slotted spoon, drain for half a minute 
and drop it in the yogurt sauce. Scoop over the sauce on 
the vadas. When the second batch of vadas are ready to go 
into the yogurt sauce, remove the first batch vadas from 
the sauce and place them on a wide platter.
Repeat for all the batches and in the end, pour any left 
over yogurt sauce on top of the vadas.

These taste best right away.

You may store the vadas dipped in water for about three to 
four days in the refrigerator. However, if you soak them in 
the yogurt sauce, then they will not stay fresh for long as 
the yogurt sauce turns sour. So, best to store only vadas 
dipped in water.