Step 1Wash the dal and rice separately until the cloudiness in the water disappears. Soak them separately in tepid water to cover at room temperature until the dal expands and softens and the rice takes on a white opaqueness, about 4 hours. Drain and reserve the liquid.
Step 2Grind each separately in a blender, using enough of the reserved liquid to make a smooth consistency. The batter for both should be very thick, so add as little water as possible. To check the consistency, take a small amount of batter and rub it between your index finger and thumb. You should feel no grittiness.
Step 3Empty both batters into an oversized bowl (the batter will double when fermented), using more of the reserved water to wash out the stubborn bits stuck in the blender. The batter should now be of the consistency of pancake batter that is neither runny nor lumpy when dropped. Add the salt and mix thoroughly.
Step 4Cover the bowl loosely with plastic wrap and place it in a warm oven (For a gas oven, heat from the pilot light should be enough; for an electric oven, heat at 225 degrees 5 minutes then turn it off. A temperature of 85 to 90 degrees is essential to the fermentation process.). Leave it to ferment overnight (at least 8 hours). South India feels like an oven most months of the year, so one isn't really needed there for this purpose.
Step 5The next day, remove the bowl from the oven and mix thoroughly again. It should have doubled in volume and become thick and foamy. It should also smell slightly sour. The batter is now ready for frying; it can be covered and refrigerated at this stage if the dosas are not going to be prepared at once.
Step 6Heat a nonstick griddle or frying pan over medium-high heat. Lightly oil the surface, seasoning the pan. Blot off any excess oil with a bunched-up paper towel.
Step 7Ladle 1/4 cup of batter into the center of the hot pan or griddle and immediately, beginning at the center, spread the batter out in a circular motion, using the bottom of the ladle, until the batter is spread out thinly. Spoon a little oil around the edges and cook, covered.
Step 8Uncover after about 30 seconds. Small holes should have formed on the surface of the dosa and its edges should have separated from the pan. Leave for 30 more seconds before lifting one side gently and folding it over the other. It should be crisp at this point. To get it crackly-crisp, add more oil around the edges after spreading the batter.