The basic idli is two parts rice to one part urad dhal, both soaked separately, drained, then ground separately. The urad dhal must be very smooth, the rice should be slightly gritty, like coarse semolina. The fermentation process (which can take from 12 to 30 hours depending on weather conditions) is what makes the batter light and foamy.
250 g/8 oz rice
125 g/4 oz urad dhal
1/2 teaspoon salt
1/4 teaspoon baking powder
Soak rice and dhal separately in cold water to cover for 6-8 hours. Rinse both twice, then grind each one separately to a smooth, thick paste. Combine, mix in salt, cover and allow to stand overnight or as long as necessary to form small bubbles and develop a nice sour-dough smell. Stir in just enough warm water to give the consistency of a batter thick enough to hold its shape in the spoon.
If you have the special idli steamer (four rounded depressions in each layer of the tiered arrangement) place wet muslin on them, then put a generous tablespoon of batter on each depression and steam over boiling water for 12-15 minutes. Peel off muslin and serve hot.
Recipe excerpted from Encyclopedia of Asian Food by Charmaine Solomon (Periplus Editions, 1998)