Dry roast the cowpeas for few seconds. Add water to it and cook on medium flame till it is well cooked. Drain and keep aside
Heat oil in a pan. Add mustard seeds, curry leaves, green chilles and saute till the mustard starts to splutter
Now add asafoetida and ginger and saute for few seconds
Add water and let it boil
Now add the cooked cowpeas, coconut and salt and let it boil together
When the water starts to boil, reduce the flame and add the rice flour, cook on low flame, stirring all the time. Add more water if necessary. Cook till everything comes together. Switch off the fire. Transfer it to a plate and let it cool
Pinch small balls of the dough. Grease the steamer plate or idli plate. Grease your palms. Roll the ball in between your palms. Flatten it slightly. Make a depression at the center. Arrange on steamer plate and steam for 10 minutes
Take each one out carefully and let it cool
Offer it to the lord along with vella adai and butter, say the prayers and then partake it