Soak the Urad (Black Lentil) for around 40 minutes. The soaking time will vary depending upon the weather but I think up to 1 hour should be more than enough. That way, its not only easier to grind it to a good consistency but also ensure that they do not absorb oil while frying.
While that's soaking, grate the cabbage. Pls refer Tip 2 for the quantity to be used.
In a plate, mix this cabbage with about 1/4 tsp salt and set aside for around 10 minutes. This is done to bring out excess water from the cabbage so that the consistency of our batter does not become too liquidy later.
Squeeze out the excess water and set aside.
Prepare the oil for deep frying at this point. Now drain the urad dal very well making sure that no water is left in the dal.
Grease a Banana leaf/plastic zip lock bag/flat plate and set aside.
Place the well drained lentils in a food processor.
and grind it without adding much water. Depending upon your blender, you might need varying amount of water to be able to grind this but make sure you add the water teaspoon at a time. This minimal addition of water is extremely important when it comes to shaping the vadas.
Use the Pulse option in your blender to grind it to a smooth consistency - should be thick but smooth.
Add rest of the ingredients.
Give it a good mix and shape a small portion into a small ball.
Flatten it on the greased surface and pat it down to make a disc. You can oil your fingers to avoid the batter sticking to your hands. Or you can use a small bowl of water on the side and wet your fingers every time it starts sticking to your fingers.
Note: Make sure to prepare these vadais as soon as the batter is made. Fermenting the batter will result it soggy fritters. Though, in the winters of US, I do not face this issue as much given the lack of humidity. But if you are not going to be preparing the vadais pronto, simply refrigerate them promptly until frying time.
Poke a hole in the center with your finger
You can wet your finger with little water/oil to aid you to make the hole.
Transfer this to your hand (or if skilled enough, slide it directly, but slowly, to the hot oil.
Make sure to keep the oil temperature to 350F- 375F. Too hot will brown the outside faster without cooking the insides and too less heat will result in a soggy Vadas.
Depending on the space in your deep fryer, you can fry 1 or more vadas at a time. Make sure to keep regulating the oil to the temperature mentioned above.
Fry until browned on the outside. If fried properly, your vadas/fritters wont absorb any oil.
Drop it on a paper towel.