Heat a pan and dry roast urad dal, chana dal and tur dal separately till golden brown and keep aside to cool. Roast over low flame to avoid the dals from getting burnt.
Heat a tsp of oil in the same pan and roast whole dry red chilies till it changes color and transfer to another plate. Add another tsp of oil and first roast whole black pepper corns and drain onto the same plate.
Add coriander seeds to the same pan and roast till aroma arises and the color changes slightly and keep aside.
Add curry leaf and sauté over low flame till the moisture completely evaporates and changes color. Add the remaining 1 tsp oil and roast till crisp and keep aside.
In the dry mixer grinder jar, first add all the dals and grind till coarse.
Add roasted coriander seeds, dry red chilies, pepper corns, peanuts and grind till coarse.
Add the roasted curry leaves, salt and asafetida (if using) and grind to a fine powder.
Remove the idli powder on to a plate and let it cool down completely.
Transfer to a dry jar and use a dry spoon to serve.
While serving mix a tsp of idli powder with gingelly oil and serve with hot idlis or dosas.