Soak the urad dal in a bowl of water overnight.
The next day, drain the dal, transfer to a blender or food processor and process to make a smooth paste.
Put the ground dal in a clean bowl, add the semolina, and set aside overnight.
Meanwhile, soak the rice in another bowl of water for 15 minutes.
Drain the rice, transfer it to a blender or food processor, and process it to make a smooth paste. Put the ground rice in a clean bowl and set it aside overnight.
The next day, line idli moulds with a damp cloth.
Mix the ground dal mixture and rice together. Add the baking powder and season with salt, then whisk for about 2 minutes.
Put spoonfuls of the mixture in the prepared idli moulds and steam in a steamer or pressure cooker for 8 - 10 minutes, or until cooked.
To check if the idlis are done, insert a cocktail stick (toothpick) into the middle and if it comes out clean, they are cooked.
If you use a pressure cooker, don't apply the weight or whistle.
If you don't have idli moulds then use small heatproofbowls, such as ramekins. Oil them lightly beforefilling them with the rice and dal mixture and steamingthem in either a double boiler or an egg poacher.
Allow to cool for 1-2 minutes, then remove the idlis and serve with Sambhar and karpodi.