In a mixing bowl, add 3 cups of rava, 1 cup of besan, salt, 1 tsp sugar, and mix well.
Gradually add 3 cups of buttermilk and whisk to make a smooth, medium-thick, pouring-consistency batter.
Cover and let it rest for 15-20 minutes to allow the rava to puff up.
After resting, add 1 cup of water (or as required) to adjust the batter to a medium-thick, ribbon-like pouring consistency.
Now add ½ tsp of baking soda and a little water. Mix well.
Heat a tawa, sprinkle some water on it, wipe it with a cloth to cool it down slightly.
Pour 1.5 ladles of batter onto the tawa and spread it evenly.
Apply oil, and if needed, remove some excess batter for a crispier dosa texture.
Add some butter to the dosa and roast it over medium heat until crispy.
The crispy paper dosa is ready to be served hot.