Pour warm water into a small bowl, sprinkle with yeast, and let stand until foamy, about 10 minutes.
Sift flour into a large bowl; add the yeast mixture along with 2 cups water and whisk until a very smooth batter has formed. Cover tightly with plastic wrap and let sit at room temperature for 4 hours.
Briskly whisk the bubbling batter until smooth; add salt and whisk to combine.
Heat a 6" nonstick skillet over medium-low heat. Pour in 1⁄4 cup of the batter, tilting and swirling the skillet to coat it evenly with batter. Cook until just set, about 1 minute. Cover and cook until the edges pull away slightly from the sides, about 1 minute more. Using a heatproof rubber spatula, transfer injera to a paper towel-lined plate, cover with another paper towel, and repeat, stacking the flatbreads as you go. (Whisk the batter while each flatbread cooks.)
Let injera cool completely; wrap with plastic wrap until ready to serve. (The injera may be stored overnight in the refrigerator; bring to room temperature before serving.)