How to Make It
Rinse the urad dhal in several changes of water, then soak it for 4 hours; drain.
In a food processor, combine the dhal with 1/2 cup of the water and pulse until finely ground.
In a large bowl, mix the yeast with 2 tablespoons of the water and let stand until foamy, about 10 minutes. Add the ground dhal, rice flour, salt and 3/4 cup of the water and whisk until a smooth, thick batter forms. Cover loosely with plastic wrap and let stand in a warm place until foamy and doubled in bulk, 12 to 18 hours. Add the yogurt and the remaining 1/2 cup plus 2 tablespoons of water to make a pourable batter.
In a small bowl, combine the vegetable and sesame oils. Heat a 10-inch cast-iron skillet over moderately low heat. Dip a paper towel in the oil and carefully wipe the inside of the skillet. Using a ladle, stir the batter and pour 1/4 cup into the center of the skillet. Working quickly, spread the batter out evenly to make an 8-inch pancake; move the back of the ladle in a spiral motion from the center. Fill in any large holes with a little extra batter. When the top looks dry and the underside has browned, about 1 minute, turn the dosa and cook until lightly browned, about 30 seconds. Transfer to a platter and continue making dosas, wiping the skillet with oil each time. Serve as soon as possible.