Soak rice and black gram separately in water for 4 hours. Then wash with water several times. Grind black gram in a wet grinder with 1/4 cup water. Grind rice in the same manner. Mix both flours, then add a little salt. Allow the mixture to ferment for 12 hours. After fermentation it is ready for making dosai.
Heat a nonstick pan over medium flame. When the pan is hot, brush it with vegetable oil. Spread 1/2 cup of the mixture on it, then cover it with a lid. After 2 minutes, collect the dosai.