Soak black gram by simmering in water for at least 4 hours or until its soaked well.
In wet grinder add soaked black black gram after removing the water. to this add 1 green chili, 1/2 inch ginger, 1/2 teaspoon (tsp.) salt. Grind these to a fine thick dough by adding little water possible. NOTE: if you add more water the soft vadai will soak lot of oil while frying.
In mixer add shredded coconut, 1 green chili, 1/2 ginger piece, 1/2 teaspoon (tsp.) of salt and grind to fine coarse and add to yogurt or curd and mix well. In a pan heat oil and add mustard seeds. Once the mustard seeds stop splattering add this to yogurt or curd 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.
Add all the vadai to yogurt or curd. Once soaked well serve with little yogurt or curd and garnish with coriander leaves.
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 and drop it in oil.