Soak urad dal in water overnight or for 4-5 hours. Drain it and grind well to a smooth paste by adding little water.
Add the chopped ginger, green chillies, cumin seeds, asafoetida powder along with salt. Stir the batter for a minute to make more light and fluffy, keep it aside.
Heat oil in a pan, add spoonfuls of the batter in it. Fry the vadas till it turns golden and crisp. (both sides )Drain on paper towels.
Take water in a bowl , add the vadas to it Soak them for 10 -15 minutes.
Take each vadas and press gently between your palms to remove excess water and keep it in a bowl or plate.
Whisk yogurt till smooth. Pour the yogurt over the vadas.Top with green chutney, tamarind chutney as required.
Dahi Vada is ready to serve. Sprinkle chilli powder, garnish with coriander leaves.