Soak urad, kambu + idli rice for 3 hours.
First grind urad dal until smooth and fluffy.
Add ice water gradually while grinding to get it light and airy.
Remove in a mixing bowl.Then grind kambu + idli rice with salt and little water to a smooth batter.
Add to the ground urad dal and mix well. Set aside for 4 hours minimum.
To make dosai, heat a tawa and grease with oil. Do not pour, just smear it with oil.
Mix the dosa batter well and pour batter in it. Spread into slightly thicker dosai. Drizzle oil if needed.
Cook in medium flame and wait until 2 mins to make it golden.
Do not cook in high flame to make it fast. It is important to cook in medium flame.
Flip and cook 1 more minute. Carefully take it out.