I feel that apart from making the batter, how you steam it also will make the difference. In India and in Tamil Nadu people have special vessel made of copper with water filled at the bottom. Inside the vessel, the idli plates will be placed with a thin white cloth spread and the batter will be filled. The vessel will be heated in medium fire and half way through, they used to test whether the idlis have been steamed properly by inserting the butt of the match stick. Here, I apply slight oil on the idli plates and after keeping in a vessel over the gas, I have found optimum time for stopping the steaming. I grind idli rice soaked for over 10 hours and 1/4 of udad dal (not 1/5 as given here) for half an hour. Grinding udad dal properly is the secret for getting soft idlis. If it is ground properly, I have found the paste almost flying. As for grinding rice, it need not be ground to a very fine paste like udad dal and if it is slightly coarse it gives very soft idlis.