Soak ¼ cup of urad dal in warm water for 30 minutes or so.
Grind soaked urad dal with no to least water to a thick paste.
Add that urad dal paste to fermented batter and mix well. Taste it and add more salt/spice if needed. Add more water if the batter got too thick during fermentation process. Again, the consistency should be thicker than cake batter.
Heat up oil for deep frying.
Once oil is hot, start dropping small balls (smaller than golf balls) of batter in hot oil very carefully. The heat should be on medium or just a bit more than medium. With right amount of heat, the vada should start coming to come the top of the oil and float while frying. Low heat with not hold the vada together and the batter will all fall apart in oil. Too high heat wont cook them from inside.
Keep frying them on medium heat while turning them few times to make sure all sides get good brown color.
Once vadas are nicely golden brown all around, take them out and place them on a paper towel. Once it is a little cool enough to hold and break, go ahead and break one in half. See if the inside is cooked by touching it and if it the batter does not stick to your finger along with some nice net like texture inside, the vadas are perfectly cooked.
If by any change the vada is a little uncooked from the inside with cooking them for decent amount of time on medium heat, you need to add some hot oil to the batter and mix it. Add 1 tbsp at a time and do the same frying/opening/touching the batter test until you get it right!
Deep fry all the vadas same way. It is best to place them in brown bag to soak up extra oil.
Serve them with some kheer/doodh pak or chai along with masala puri! They stay good for up to a week in the fridge or up to 3 days at room temp.