I had posted this recipe on ehow, so I am actually pasting the same here.
Ehow article: http://www.ehow.com/how_5204800_make-adai-vadai.html
How to Make Adai Vadai
Adai vadai is a deep fried snack in south india. It is crisp and tasty.
Things You'll Need:
* 2 cups rice - soaked for 2 hours
* 1 onion - chopped
* 4 red chillies
* A strand of curry leaves
* 1/4 cup coconut
* 2 tbsp cumin seeds
* 1 tbsp coriander seeds
* Blender, wok for deep frying, plastic sheets / clingfoil for patting the vadai
Wash rice well in water.
Soak the rice in water for at least 2 hours.
Drain the water completely. This is important for getting a thick batter
Fry the chopped onions without oil, to dry them out a little. Wait till the onions cool.
This step can be omitted as you get more experienced.
In a blender, add all the ingredients except the rice, and blend till smooth. Wait a minute before opening the blender, or the spice from the chillies will burn your eyes.
Transfer the paste to a dish, and now add the rice to the blender, and blend away, till it is slightly gritty.
Now knead both ingredients together till mixed well.
Add a few drops of water, till you can break off a piece and pat it into a slightly moist patty with your hands.
Heat the oil till it is moderately hot. To test, put a small piece of the batter in the oil. It should immediately rise to the top and get browned.
With a round spatula with holes, take this piece and lay aside.
Spread the clingfoil on the tabletop. Take a small piece of batter, smooth it between your palms into a ball. Place this ball on the cling foil, lift an end of the cling foil and place above the ball, so it is covered. Now flatten out the ball with your palm. The size of the ball should be small enough that when pressed, it is no larger than the square of your palm.
The pressed vadai should not be too thin, else it would not puff up.
Cling foil can be avoided, if you just pat it into that size in your hands.
Gently drop this vadai into the oil, from the side of the pan. Wait till it puffs up and appears slightly browned. Turn the vadai and let the other side fry. When done, use the spatula to take the vadai, hover over the pan a few minutes to let the oil dribble, and place the vadai on a plate covered with 2-3 layers of kitchen tissues.
Serve when hot!
Tips & Warnings
1/4 cup of lentils (toor dal) can be soaked with the rice for added flavor. Lessen quantity of rice accordingly.
Wearing an apron is optimal, as you can avoid splatters on your clothes
It is best that other items are cleared from the stove top, and that people are not walking often around the cooking area, so you can move about without fear of splashing oil.
Utmost care to be followed when deep frying, as oil can splash and cause burns.