Add both the rice flour and urad dal flour into a mixing bowl.
Pour water and mix well. The batter has to be smooth and without any lumps and evenly mixed. The consistency should be that of a thick batter but still flowing.
Pour the batter from the mixing bowl in another bowl. Cover and allow to ferment at room temperature overnight for 9-10 hours or more if required.
Once the batter is fermented well, then add salt to the dosa batter and mix well.
Heat an iron griddle or tava. Smear with oil lightly.
Take a spoon or ladle with the batter and gently start spreading the dosa in circular rounds. If using non stick pan, then avoid smearing oil on the pan. Since you won't be able to spread the dosa batter properly on the non stick pan.
Drizzle oil on the top and sides and let one side cook.
When one side of dosa is browned, flip and cook the other side.
Fold the dosa and serve hot with coconut chutney or vegetable sambar.
Make dosa in the same way with the batter. The leftover dosa batter can be refrigerated and used within 4 to 5 days.