Soak 1 cup of urad dal and 1 cup of yellow moong dal in enough water for 4–5 hours. Do not over-soak.
Drain the water and grind the dal into a thick, smooth paste, adding water in batches. Use about ¾ cup of cold water. Transfer the dal paste to a large mixing bowl and beat the batter in a circular motion until it becomes light. This helps incorporate air into the batter and makes the vadas soft and fluffy.
Cover and rest the batter for 15–20 minutes. Resting the ground batter allows slight fermentation.
Then add salt, a pinch of hing, 1 tsp cumin seeds, and 1 tsp green chili paste. Mix well.
Wet your fingers and shape the vadas into round balls.
Heat oil and gently drop the vadas into it. Deep-fry on a medium flame, turning occasionally, until they turn golden brown and crisp on both sides.
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.
Once the fried vadas have completely cooled, drop them into the water and ensure they are fully submerged. Soak for 10 minutes or until they absorb the water. Then gently squeeze out the excess water and transfer them to a plate.