I'm not familiar with rasam powder, I used to buy from market or either use chilli pdr, coriander pdr pepper and jeera separately. Now I just roasted all these together and worked out well.



Ingredients:
        
Red chillies                : 20nos
Coriander seeds         : 1/2cup
Peppercorns               :1/4 cup
Cumin seeds              : 1/4cup
Asafoetida                 :1"pc
Chana dal                  :1/4cup
Tur dal                       :1/4 cup



Method:

Dry roast all the ingredients except asafoetida till the dals become light brown in colour. Remove from fire, let it cool. You must add the cumin seeds at the final stage only.



Take a teaspoon of oil and add asafoetida and fry this. You can also use asafoetida powder. In that case no need to roast or fry it. You can directly add this to the rasam.
Dry grind all this together to a fine powder.
Store it in an airtight container.