The cakes are made by steaming a batter consisting of fermented de-husked black lentils and rice.

Vaddaradhane, a 920 CE Kannada language work by Shivakotiacharya, mentions "iddalige", prepared only from a black gram batter.

Chavundaraya II, the author of the earliest available Kannada encyclopedia, Lokopakara (c.1025 CE), describes the preparation of this food by soaking black gram in buttermilk, ground to a fine paste, and mixed with the clear water of curd and spices.

[1] The Western Chalukya king and scholar Someshwara III, reigning in the area now called Karnataka, included an idli recipe in his encyclopedia, Manasollasa (1130 CE).

In Karnataka, the Idli in 1235 CE is described as being "light, like coins of high value", which is not suggestive of a rice base.

According to him, the cooks employed by the Hindu kings of the Indianised kingdoms might have invented the steamed idli there, and brought the recipe back to India during 800–1200 CE.

[8] The earliest extant Tamil work to mention idli (as itali) is Maccapuranam, dated to the 17th century.

Optionally spices such as fenugreek seeds can be added at the time of soaking, for additional flavour.

However, this varies greatly by region and personal taste, it is also often served with kaara chutney (onion-based) or spicy fish curries.

[18] In addition to or instead of fenugreek, other spices may be used such as mustard seeds, chili peppers, cumin, coriander, ginger, etc.

Fermentation of idli batter results in both leavening caused by the generation of carbon dioxide as well as an increase in acidity.

Hence the salt in the batter and the ongoing generation of lactic acid both suppress the growth of other undesirable micro-organisms.