Soak 1 cup of urad dal and ½ cup yellow moong dal in enough water for 4 hours. Do not over-soak it.
Remove the water and grind the dal into a thick and smooth paste by adding water in batches. Use ½ cup of cold water. Transfer the dal paste into a large mixing bowl and beat and mix the batter in a circular motion until it turns light. This helps incorporate air into the batter and makes the vada soft and fluffy.
Then add 1 tsp cumin seeds, 1 tsp chopped green chilli, 1 inch chopped ginger, salt, and a pinch of hing. Mix well.
Now wet your finger and shape the vada into a round shape.
Heat oil and drop the vada in the oil, deep-frying the vada on medium flame. Fry on both sides until they turn golden brown and crisp.
In a large bowl, take 5-6 cups of slightly hot water. Add ½ tsp salt, ¼ tsp hing, and some coriander leaves. Mix well, ensuring it is well combined.
Drop the hot fried vada into the water and dip completely. Soak for 15 minutes or until the vada absorbs water. Then squeeze off the water and transfer to a plate.