I made Lori’s Chocolate Midnight Cake by Samin Nosrat.

Is there a chance your baking soda has expired? I got caught by that once. Make that twice. Scientific method, you know.

I used half water and half coffee. I’ve made this cake successfully several times. I don’t know why it was a dud this time.

I used Cacao BarryExtra Brute, which is what Stella Parks recommends. The label doesn’t say Dutched but it does say alkalined. I made Bravetarts Devils food cake today and it turned out great.

What kind of cocoa did you use?

I believe the only leavening agent in that cake is baking soda. When you use soda, you have to be fairly quick getting the product into the oven, because the chemical reaction which produces rise begins happening the minute it comes into contact with the acid. Your pan height would not affect it, as the actual width is still the same 8 inches. I think you just had a delay in baking which let too much gas escape before the structure of the cake set enough to trap it. It's also possible that your batter was too thin, which would also let the gas escape more easily. Providing your measurements were accurate, it may be due to having flour which was a bit damp, and unable to absorb as much liquid as it normally might. On humid days, it may require a bit of tinkering- sometimes just adding another tablespoon of flour to adjust a batter thickness will do the trick. Otherwise, when you are working with baking soda, you need to have both pan and oven ready to go, mix quickly and get it into the oven. Otherwise, you do get flatter results.