Soak 1 cup urad dhal in 2 cups water for 5 to 6 hours
We are making the vadai in a mixing jar/ blender and not the traditional grinder.
After 5 hours, strain the dhal and save the water (do not waste it). Keep the soaked dhal and the water in refrigerator for 30 minutes
In the meantime, prepare the yogurt mixture. Whisk 3cups yogurt, add salt. Heat oil in pan, add mustard seeds. Once it begins to splutter and curry leave. Add the to the yogurt mixture and mix well. Keep aside
Now grind the urad dhal with salt. Use the refrigerated water one tbsp at a time. Do not too much water. Too much water will made the vadai absorb more oil when frying. Grind smooth and fluffy.
Now the trick to the perfect vadai, and fluffy inside, you have to whisk them and incorporate air into the batter. The batter should be light and fluffy. Take some water in a bowl, drop a little batter. It should float up. Then batter is ready!
Keep a container filled with hot water ready.
Heat oil for frying.
Fry the vadai in batches. Dip your hand in some cold water and then pick the batter.
You can made vadai with or without the hole in the middle. My mother makes without the holes.
Fry till golden brown. Drain the vadai in paper towel for a couple of minutes.
Then add the vadai into hot water and let it soak for 8-10 minutes. Change the water every batch to get rid of the oil. You will notice the vadai would have swelled up.
Now take out the vadai and gently squeeze between you palm and place in the yogurt mixture.
Let the vadai be covered with yogurt. Let it rest for 20 minutes to full soak in the yogurt.
Garnish with coriander and boondhi. Soft and Moist Thayir Vadai is ready!