Wash and then soak urad dal, yellow moong dal and green gram split moong dal for 8 hours or even overnight.
Drain and transfer to a blender jar. Also add broken green chillies. Then add ginger paste, salt, coriander powder and cumin powder.
Grind to make a smooth and thick batter by gradually adding cold water. It should not be grainy in texture but smooth.
Transfer the batter to a bowl. Add baking soda and hot oil. Whisk the batter till it's light and fluffy to aerate. The vada batter is now ready.
Add oil to a frying pan. Keep the flame on medium. To check if its hot enough, add a small pea-size batter to the oil. If it immediately floats up to the top, that means the oil is very hot, lower the flame. If it sinks to the bottom, then it means oil is not hot enough. The right temperature would be when the ball takes a couple of seconds to float up.
Using a small spoon or tablespoon, drop the balls in oil. Fry them on medium flame till they evenly turn golden brown. Keep tossing them in oil to cook evenly. Fry them in batches. Do not overcrowd.
Transfer the balls in warm (not hot or room temperature) water immediately. It should float. Leave them in water till they become bigger, turn soft and fluff up or leave it for 15-20 minutes.
Squeeze out the excess water from them using your palm. Press gently so that they don't loose their shape and break. Transfer to a plate for now.