Place the dal lentils and the rice in two separate containers. Cover each container with water and soak for 4 hours.
After 4 hours, drain each container, conserving the soak water from the rice. Discard the soak water from the lentils.
Place the soaked lentils in your Vitamix, Blend-tec, food processor or blender. Add water to cover the dal and blend until smooth. Note: the dal MUST be very smooth. It cannot be grainy in texture. Transfer to a very large container.
Place the soaked rice in your Vitamix, Blend-tec, food processor or blender. Add enough of the soak water from the to cover and blend until smooth. (If your blender is small, process the rice in two batches, adding half of the rice and water each time). Transfer the smooth mixture to the same large container with the processed lentils.
Mix the smoothly processed lentils and rice together to create the dosa batter. Add 1 ½ teaspoons of salt. Ferment the mixture on the counter for 12 hours, or until the mixture rises and begins to bubble.
The fermented mixture will increase in volume. It will most likely not double, but you may see a 25% - 30% increase. Make sure to use a container that is large enough to accommodate the batter’s expansion. Once the batter has expanded, put the dosa batter in the refrigerator. (If you don't refrigerate it, the batter will keep expanding!)
Once the level of the mixture is higher than what you started with, and the mixture is regularly bubbling, (like pancake batter), you’re ready to make the dosa! You can either prepare the dosa now or store in the fridge until you're ready to make dinner.
Heat a non-stick griddle or pan to medium. Once the pan is hot, spread a few drops of oil with a folded paper towel. Scoop one ladle of batter into the hot pan and immediately spread it in a circular motion. It must be very thin – like a crepe. It will need to cook for about 2 – 3 minutes, depending on the thickness of the dosa. While the dosa is cooking, you may want to add a bit of oil around the edges to help make it easier to lift out of the pan (optional). Let it cook until the dosa is golden brown.
When the top side of the dosa turns off white in color, (the side facing you), the dosa is cooked. If the top side remains white, check the bottom side. Once the bottom turns golden brown, flip the dosa until the other side turns off-white.
Use a spatula to loosen the dosa all round its edges, lift it from the pan and serve it immediately. You may fill with the potato masala or just eat it plain. Serve with chutney for the true south Indian experience.
After each dosa is cooked, take the folded paper towel and wipe the pan to cool it a bit before adding more batter. Any unused batter may be stored in the fridge for up to a week.