Soak rice and flattened rice, separately, in enough water for 3~4 hours.
Drain the water completely.
Grind them separately, using buttermilk/ sour yogurt, into a smooth batter.
Pour the batter into a large bowl and keep in a warm place to ferment. In a warm house, this might take anywhere from 6~8 hours. But in cooler regions, this might take up to 12~15 hours, so plan accordingly.
Well fermented batter will increase in volume and will be airy and lots of bubbles. It will smell tangy and fermented. This indicates it’s time to make dosa.
If the batter looks too thick, stir in a little buttermilk to get the right consistency. Also stir in salt.
Heat a nonstick or cast iron tawa on medium-high heat. Pour about ½ cup of batter on the hot tawa and using the back of the spoon spread it evenly into a thin dosa.
Drizzle generously with oil and cook for 1~2 minutes or until the sides start to turn brown and lifts off from the pan. Gently flip with a thin spatula and cook for 30~45 seconds on the other side.
Fold in half and remove from the pan. Serve immediately with chutney of choice.