Stir in flour, espresso powder, and salt until just a few streaks of flour remain.
Stir in chocolate chips till flour is fully incorporated.
Spread the dough into four 5" cast iron skillets (or spread a quarter of the dough into a 5" cast iron skillet to make one at a time).
Bake at 350° F for 20 minutes. Let cool for 10 minutes before serving.
Notes
The salt measurements in this recipe are for Diamond Crystal Kosher Salt. If you are using a different salt you will likely need to reduce the measurement.When adding the flour, you don't fully mix because you will be mixing again when you add the chips. Leaving the streaks of flour and fully incorporating once the chips are added prevents the batter from being over-mixed.To instead make in two 6.5" skillets, bake for 30 minutes and cool for 10 minutes.To instead make in one 8" skillet, bake for 35 minutes and cool for 15 minutes.