Wash and soak channa dal for 1 hour or until it is soft. Drain the water and set aside.
In a large bowl, mix rice flour, urad dal flour, asafoetida, red chilly powder, sesame seeds, curry leaves, chana dal and salt.
Add the butter and mix with hands into the flour until well incorporated. Add warm water little by little to form a stiff and smooth dough. Cover and set aside for 5 minutes.
Heat oil for deep frying. Meanwhile divide the dough into gooseberry-sized equal balls and keep them covered.
Take two zip-lock or milk covers and lightly grease one side with oil. Take one ball and place on the first cover and place another cover on top of it.
Press with a flat-bottomed vessel like davara or steel glass. Then flatten using fingers as thin as possible. Prick with a fork to prevent puffing.
When oil is hot, carefully take the thattai and slide into the oil. Cook on low-medium heat until the thattai becomes golden and crisp and the bubbles cease.
Remove and drain on a tissue paper. Once cool, store in an airtight container.