Soak Urad dal for 45 min to 1 hr only. Do not soak it longer then the vadas will soak in too much oil while frying.
Drain the dal and grind it into a very fine/soft thick batter. Use very little water while grindiing it.
Then add the salt, coriander and chilles to the batter.
Get a frying pan ready with oil. Put a small piece of batter in the oil to test it before you start making the rest of the vadas.
Oil your palm slightly, place a dollop of the batter on your palm, use your thumb to make a hole in the center ( like a doughnut) and then slide it into the frying pan.
Serve hot and it should be crunchy. You can eat it with chutney or sambar.