Preparation method:

1. Wash and soak black gram/ulunthu and fenugreek seeds together in a wide bowl for at least 2 to 3 hrs or over night. Same way wash and soak idli rice in a wide bowl for at least 6 to 7 hr or overnight. 
2. Put some ice cubes in water and use that to grind batter. Grind soaked black gram in a wet grinder (I always prefer wet grinding for idli and dosai) until it is light and fluffy by adding water little by little and transfer it to a container.The batter should not be sticky but it should be light and fluffy. I usually grind ulunthu for about 40 mins. 
3. Now add soaked rice to the wet grinder and grind it coarsely by adding water little by little for 10 to 12 mins to a slightly coarse paste. Consistency of rice batter should not be too grainy nor fine paste. 
4. Transfer the rice batter to the same container in which urad dal batter is. Then add the required salt, using clean hands mix the batter thoroughly for 10 to 15 minutes, so that it gets mixed well. 
5. Container should have enough room for the batter to ferment. Consistency of batter should not be too thick or too thin. Ferment the batter for overnight or at least 9 to10 hrs. 
6. Mix the fermented batter once ( do not try to stir or mix too much). Now it is ready for making idli. If you are not going to use it immediately, then refrigerate the batter.

Making of soft idli:

1. Heat enough water in the idli pot, grease the idli platter with little oil or ghee (for kids) and then take a scoop of batter and fill in the idli platter. Once you see steam from the idli pot, place the idli platter and close it with a lid. 
2. Steam for 9 to 10 mins. Using fork or toothpick you can check whether it is done or not. If it is done, then remove platter from the pot . Either allow them to cool for 5mins or you can turn over the idli platter and show it in the cold running water. 
3. Remove idli from the platter and transfer it to a hot pack. Spongy idli is ready and now serve with sambar,chutney and idli podi.

Notes:

• I use either ice water or ice cubes while grinding idli batter, that helps the batter from getting hot. 
• The quality of idli rice and black gram/ulunthu is very important. Never stock rice and ulunthu for long, better get it in small quantity or when needed. 
• The duration for fermentation may vary depending on the climatic conditions. In winter it may need longer time and in summer it may require less time to ferment. Always try to keep it in a warm place. 
• The consistency of the batter is also equally important. If your batter is too thin, then your idli will be flat. If your batter is thick, then your idli will be hard. That is the reason why you should add water little by little by checking the consistency. 
• You will get spongy idli only when your batter is nicely fermented. You will see lot of small bubbles in your batter and never forget to mix your fermented batter before pouring into the idli platter. 
• You need to take out the batter from refrigerator about 30 mins before you make idli. Mix well once before pour into idli platter. 
• You may also make crispy dosai as well as spongy uthappam with same batter.