Wash urad dal several times in water and soak for 2 hours in the refrigerator.
After 2 hours drain the water from urad dal and grind smoothly adding little water intermittently. Urad dal must be smooth and fluffy.
Take the urad dal batter in a wide bowl and add finely chopped onions, green chillies, curry leaves, coarsely ground black pepper, finely grated ginger and salt. Add one tablespoon of rice flour.
Mix everything together and beat the batter with your hand continuously so that air is incorporated into the batter.
This step is crucial since proper beating brings out the fluffiest and crispest vadas.
Now heat a pan and add oil for deep frying.
Once the oil is hot, wet your fingers, take a little bit of batter, make a hole with your thumb and gently place inside the oil.
Once evenly browned, flip over the vadas and cook.
Strain and serve hot.