Although I did not manage to get the perfect shape, the vadais were deliciously crispy.
1 cup ulundu (urad dhal)
1 tsp fennel seeds
3-4 green chilis
4-5 springs of curry leaves
Salt to taste
Oil for deep frying
Soak the urad dal for 3-4 hours. You need to remove the black skin first (if you use urad dal without skin you can soak it for 2 hours). If you pour cold water and make circular motions in the water with your hand, you'll notice that the skin comes off and floats. Wash out the skin. You'll have to repeat this process until all the skin has been washed away. Cut the onion and the green chili.
Now place half of the urad dal in a blender/grinder and grind the dal very smooth and thick. Add the least amount of water you possibly can. If the batter contains too much water it will be difficult to "shape" it into vadais. When you have ground all the urad dal, pour the mixture in a large bowl and mix all the ingredents. Meanwhile, heat oil for deep frying the vadai. Cut some squares, large enough to shape vadai, on a clean plastic bag/sheet and oil the squares on one side. Take some batter and place them on the square, flatten them slightly and make a hole in the middle. When the oil is ready, remove the vadais from the plastic squares and place them carefully in the hot oil and deep fry both sides.
Have a plate with a tissue paper prepared to place the deep fried vadais and drain the excess oil. At first it is better to fry only one vadai and taste it and add more salt, if needed, and then after that, deep fry more vadais at a time. These vadais are best consumed on the very same day they are made, when they are still warm and crunchy. Best of luck!