firstly, soak 1½ cup urad dal for 2 hours. make sure to rinse urad dal well.
drain off the water and transfer to the grinder.
blend for 40 minutes to smooth and fluffy batter adding water as required.
transfer the urad dal batter to a large bowl and keep aside.
in a large bowl take 3 cup idli rava and rinse 3 times.
drain off the water making sure the water is running clean.
now squeeze off the idli rava and transfer to the urad dal batter.
mix well making sure everything is well combined.
cover and ferment for 8 hours or until it is well fermented.
now add 1 tsp salt and mix gently.
finally, idli batter is ready to prepare idli, uttapam, appe or punugulu.