Soak black gram for 2 hours. Drain water add salt as needed, baking powder and grind into a fine dough. Add as little water as possible (or Vadai will observe lot of oil.)
Add finely chopped onion, curry leaves, ginger, green chili and mix well.
Now heat the oil in the fryer or pan. Now take small amount of mixed dough ( a size of lemon), flatten it make a hole in center and deep fry them.
NOTE: Hardest part of this is making the dough rolled and flatten it. It will be very sticky. Solution is to dip both hands in water and make the dough ball, flatten make a hole and drop it in oil.
-- Sambar Preparation --
Soak the tamarind in 3 to 4 tablespoon of water and mash them into a thick paste.
Now add the sambar powder to the tamarind and mix well and keep aside.
In a pressure cooker add the red gram, shallots, chopped tomato, turmeric powder, cumin seeds, 1 1/2 cup of water and cook them for 3 whistle. Once the pressure is released mash the red grams into a paste.
Now add the tamarind and sambar powder mixture to the cooked red gram and cook until the raw sambar powder smell goes away (15 - 20 min) and keep aside.
Heat oil in a pan add mustard seed and curry leaves once the mustard seed stop splattering add this to the sambar mix well.
-- Serving sambar vadai --
Add the cooked vadai and coriander leaves to the sambar and mix well.
Let it soak for 10 - 15 min.
Once the vadai is soaked well it is ready to be served.
When you serve the sambar vadai you can add some fresh finely chopped onion.