Preparation of Batter

Start by soaking the idly rice, raw rice, and urad dal together in water for about 6 hours.
After soaking, grind the mixture into a coarse consistency, ensuring the rice and urad dal remain grainy.
Add salt to the batter and mix well. Set it aside in a warm place for fermentation for about 5 hours.

Tempering

In a small pan, heat oil over a medium flame.
Once hot, add mustard seeds. Wait until they start to splutter.
Add the cumin seeds, crushed pepper, green chilli, ginger, cashews, and curry leaves. Sauté till the cashews turn golden brown.
Lastly, mix in the grated coconut and sauté for another minute. Remove from heat.
Add this tempering to the fermented batter and mix thoroughly.

Final Mixing

Add the baking soda and curd until well combined with the batter.

Steaming the Idly

Lightly grease tiny tumblers or idly moulds with oil or ghee.
Pour the batter into the moulds, filling up to 3/4th of their capacity.
Place these filled moulds in an idly steamer or a large pot with a tray. Ensure there is enough water at the bottom of the pot for steaming.
Cover and steam on high flame for 13-14 minutes.
After that time, reduce the heat and check for doneness by inserting a clean toothpick into the centre of an idly. If it comes out clean, the idlis are cooked perfectly.
Turn off the heat and carefully remove the moulds from the steamer.

Demoulding and Serving:

Allow the idlis to cool slightly. Using a butter knife, gently loosen the edges of the idlers from the mould.
Carefully invert and tap out onto a serving plate.
Serve your delicious Kanchipuram Idly hot with coconut chutney and sambar for a delightful South Indian treat.

Note: The fermentation process is crucial as it gives the idly its soft texture and subtle sourness. Ensure you keep the batter in a warm place, primarily if you reside in a cooler climate.