The cup measure used here is 250 ml by volume. This much batter would make about 50 regular Dosai. You can cut down the whole thing proportionately for a lesser portion.
1) Soak all ingredients except salt in drinking water for about 5-6 hours.
2) After it is soaked, sieve the soaking water out.
3) Grind it to a smooth paste in a mixi. (Traditionally done in a stone grinder). Add some water to help grind this smooth.
4) Cover the batter, and place it in a warm place. In the summer, you can leave it on your kitchen counter and it will ferment in 4-5 hours. Kele an eye on it. In the winter, place it inside a warm casarole or inside a switched off microwave for 6-7 hours or longer, till it ferments.
5) Add salt to the batter and mix well.
6) Heat a non stick pan to medium high heat and pour a small ladle full of batter. Immediately spread it in a circular motion till it is thin and even throughout. (traditionally cast iron is used, if you have one, and you know how to use it, then go ahead).
7) Drizzle some oil around the edge and let Dosai cook for 1-1.5 minutes on one side till it is golden, then flip over and cook for about 40s on the other side.
8) Serve hot with any chutney, condiments or curries of your choice.