2/3 C split urad dal, without skins
1-1/2 C basmati rice
1/2 T salt
3/4 C butter or oil
Wash and sort the dal and soak it in 3 C cold water for 4 hours. Wash and soak the rice in 3 C of cold water, too.
Drain the dal and process it in a blender or food processor, slowly feeding warm water into it in spoonfuls until the batter is light and frothy. Put the batter into a bowl. Then process the rice into meal and slowly add water by spoonfuls until the consistency in the blender or food processor is paste-like, with no texture. Stir this mixture into the dal, cover loosely, and allow to ferment for a day. It should be bubbly.
To cook, oil a hot skillet or griddle and put 1/3 C batter on the surface. Use a ladle to spread the batter out to an 8-inch circle. Drizzle a couple of teaspoons oil or melted butter over the surface and cook for a couple of minutes. Cook it for a couple of minutes on the second side. Flip it again.
Fold the dosa in half and serve immediately.