This happened to me recently—not with this particular cake, but, suddenly, with ALL cakes, regardless of recipe author. I always keep fresh BP, so I didn’t think that was the problem. At the same time, I didn’t know what else it could be.
However, I have since purchased new baking powder, and the problem disappeared, so I can only think it WAS the baking powder, even though it was fresh and in date. Thanks for this reminder that I always want to test my BP before using:
■ To test baking soda: Put a few tablespoons of white vinegar into a small bowl and add a teaspoon of baking soda. It should bubble up furiously, and the foaming should take several moments to subside. The more bubbles, the more potent the baking soda. If there is no reaction, or you only end up with a handful of small bubbles, you need to replace you baking soda.
■ To test baking powder: Put a few tablespoons of warm water (warm tap water is fine, but cold water is not) into a small bowl and add a teaspoon of baking powder. The mixture should make a fizzing noise and, after a moment, the baking powder will begin to fizz and the water will become very cloudy with tiny bubbles. The more bubbles, the fresher the baking powder. Baking powder reacts with liquids and heat, but does not react as well with cold water (even fresh powder won’t fizz much in ice water), so do not use it for this test.
Good luck, and if you find it was something else, please let us know!!