Injera is the puffy, sour pancake of Ethiopian cuisine. It is traditionally made on a griddle the size of a small table, but we make smaller ones on a cast iron skillet.
Injera has the world's simplest sponge recipe among sourdoughs. In a mixing bowl whisk together:
Preheat the oven to just over 100 degrees, then turn off the oven. Put the bowl in the oven and forget about it until you use your oven that day or until the next morning.
Each day, for at least three days, maintain the sponge. Add a quarter cup more teff flour, perhaps some more water, whisk again, and return to the warm oven.
After three days it has fermented enough to be ready to cook, although you can wait additional days without problem. No ingredients are added: the "sponge" is now "batter".
A few tricks make the potentially tricky task very simple. Get these right and the pancakes come off easily and when you are done the griddle or skillet dusts clean with a dry rag. Ignore them and the batter sticks like crazy.
Save at least a little as a starter for the next batch. No need to add more yeast. Add teff and warm water in that two-to-three ratio until you have replenished your sponge.
Commentary: During Fall and Winter we often have the bowl of sponge/batter living in our oven for two weeks. Not only does my wife make great Ethiopian dinners, it is handy at lunch times to be able to make a few injera pancakes to go with some spicy soup or some boxed Indian food. After two weeks we start to get tired of it, use it all up, wait two or three weeks, and then begin a new batch three days before we want to use it.