1. Soak the dal in a bowl overnight. Next morning, drain the water from the dal and place dal in a blender, together with 1 cup / 225 millilitres fresh, cold water. Blend until smooth. Depending on your blender, this may require giving the dal a stir manually at intervals.
2. In a saucepan, heat 1/2 cup (100 millilitres) of water over low heat, stir in 1 tablespoon (15 millilitres) of rice flour, and continue to stir until it begins to thicken. Set aside. In a large bowl mix together the ground dal, salt, remaining rice flour, and 2 cups (450 millilitres) of water. Stir well to make a thin batter.
3. Add the thickened rice paste and stir again to mix well. Cover the bowl and let stand for 5 to 6 hours at room temperature, or as long as 12 hours if you need to. Just before you begin cooking, lift up a spoonful of batter and pour it. The batter should resemble a thin crepe batter; add more water to thin it if necessary.
4. Heat a large (11- or 12-inch / 30 centimetre) griddle over medium-high heat. (You need a griddle with low sides, for the dosa will spread to the edge of the pan and if it has high sides, you’ll find it very difficult to lift the dosa off the pan.) With a paper towel (or the cut side of a potato, the traditional way), oil the surface of the griddle lightly. When it’s hot, pour on a 1/2 cup (100 millilitres) of batter, starting at the center and moving out in a spiral shape. Use a spatula or the back of a wooden spoon to help spread the batter as far as possible to the edge of the griddle. The dosa should be made as thin as possible. Allow to cook for approximately 2 minutes on the first side. Flip over and allow to cook for one minute on the other side. Timing depends on how hot the griddle is; don't be afraid to let the dosa cook longer. It is better to have a slightly crispy dosa than an undercooked one.
5. Repeat with the remaining batter. As each dosa is removed from the griddle, stack on a plate and cover the stack of dosas with a clean cloth to keep warm.