Sponge idlis or khushboo idlis as they are popularly known after the cine star  makes excellent soft spongy idlis good for kids and elderly .

This isa  good recipe to try with different ingredients and something different from usual idlis.
The traditional recipe for Khushboo idlis use  mostly castor seeds and less of udad dal  or even skip udad dal completely . I have tasted that variety too but  I do not like to experiment with castor seeds as I have heard several controversial issues about the toxicity and it is not suitable for some to consume even in small quantities.

On a whim I added a tsp of flax seeds as I like to add a little everyday to some dishes like porridge, gravies and even some curries..That is the reason the idlis are not pure white and have brown flecks on them .This is purely optional and not required in the recipe.

Ingredients :

4 cups raw rice ( or a mix of both raw rice and idli rice ) I had only raw rice on hand so used that.
1 1/4 cup udad dal
3/4 cup sago ( javarsi )
3/4  cup aval (  beaten rice )loosely packed 
1/2  tsp fenugreek seeds.
salt as required to be added after fermentation .

( 1 tsp flax seeds purely optional )

Note :

Many  recipes add cooking soda to batter which I find is not necessary as overnight fermentation is good enough to give super soft idlis .Leave the choice to you. If you wish to add cooking soda make sure to add to fermented batter just before steaming .

Method :

Clean and wash rice and other ingredients.
Soak them all together or rice and udad dal in one container and the rest in another vessel , doesn't make a difference as they have to be ground uniformly.

Soak for 4 hours and grind to a very smooth satiny batter...there should be no grains or coarse texture to the . batter.

Grind to a thick idli batter and let it ferment overnight.

After fermentation overnight or after 8 hours , gently mix batter to make it airy, add salt and then grease idli  plates or use cloth to cover idli plates to pour batter.
Use 3/4 th ladle per each idli cup as this batter rises well and gives a fat soft idli.

Devour the soft spongy idlis with chutney or tiffin sambar.!

These idlis being very soft and spongy you are likely to eat more than normal . These idlis make excellent school lunch box tiffin and good for toddlers. and kids as it is very soft and digestible.

The flax seeds gave the tiny flecks in the idli and so not looking pure white.