Soak channa dal in water for 1 to 2 hours. Then drain the water.
If you have roasted urad dal flour, skip this step and next step. Take ¼ cup of urad dal and dry roast in a pan on low flame till you get the aroma and just turns slightly pink in colour.
Cool the roasted urad dal, and grind in mixer to a fine powder. Sieve this flour.
If you do not have roasted bengal bram flour or Pottukadalai flour, grind ¼ cup of Pottukadalai to a fine powder and sieve it.
Take a large bowl. Add all the flour - rice flour, 3 tbsp of Urad dal flour, 1½ tbsp of Pottukadalai flour and sieve well.
Now to this, add the soaked and drained channa dal, asafoetida, salt, red chilli powder, finely chopped curry leaves, room temperature butter and mix well. The butter must be incorporated uniformly in the flour.
Now adding water gradually, knead this to s smooth dough. I used roughly 1¾ cup of water for kneading the dough. This could be less or more depending on the rice flour.
Cover and rest the dough for 5 minutes.
Heat oil in a large kadai.
Now take a plastic sheet or ziplock cover and grease with little oil.
Take puri sized balls of dough.
Grease your fingers with little oil and flatten it uniformly to a thin round disc. It should not be too thick, nor too thin. It should be roughly the thickness of Puri and should not tear when you try to peel it from the plastic sheet.
Now using a fork or toothpick, prick each of the thattai at least 5 to 6 times.
Now peel off each of the thattai carefully from the sheet and drop in hot oil. Fry the thattai on medium flame.
Flip and cook the thattai till it turns pink and colour and the bubbles start subsiding. Drain the thattai from oil and let it cool. Do note that thattai would continue to change colour once taken out of oil, so do not go by the colour alone for when to take it out of oil.
Repeat making thattai as per the above steps. Once the thattai have come to room temperature, store it in an air-tight container.