Wash and soak rice and dhal separately overnight.
Wash well with plenty of water and drain it.
Grind dhal to a very fine paste along with coconut milk.
Grind rice till fine grains are left (like very fine semolina) along with coconut milk.
Mix both rice and dal together after grinding.
Mix yeast with sugar and 1 1/2 tsp warm water.
Keep it aside for 10 mins, till it rises.
Mix dal and rice, add salt and yeast and beat well.
Add a little coconut milk if necessary, the batter should be fairly thick.
Cover and keep aside for 7 - 8 hrs, undisturbed.
Spoon batter onto idli moulds.
Heat water in idli cooker, and place the stand in it.
Heat till the sannas are fluffy, approx 20 - 25 mins.