In a mixer jar, add 3 cups rava, 1 cup besan, salt to taste, and 1 tsp sugar. Grind well.
Gradually add 3½ cups buttermilk and whisk to make a smooth, thick batter with a pouring consistency.
Cover and let it rest for 15 minutes to allow the rava to absorb the liquid and puff up.
After resting, add ½ cup water (or as required) and mix well to achieve a medium-thick, ribbon-like pouring consistency. Do not make the batter too thin; otherwise, the dosa will not turn crispy.
Add ½ tsp baking soda and 1 tsp lemon juice. Mix well.
Heat a tawa, sprinkle some water on it, and wipe it with a cloth to cool it slightly.
Pour 2 small ladles of batter onto the tawa and spread it evenly.
Apply some butter and cook the dosa slightly. Then, using a slightly wet spatula, remove some excess batter to make the dosa thinner and crispier.
Add a little more butter and roast the dosa over medium heat until crispy.
The crispy and thin paper dosa is ready to be served hot.