01 -
Preheat the oven to 325°F (160°C). Line a 9-inch square pan with parchment paper.
02 -
Set the butter in a heatproof bowl and warm it in the microwave for about a minute, until it’s partially melted. Add the sugar to the bowl, whisk it in, then set it back in the microwave for another minute. The butter should be hot and fully melted, and the sugar will have begun to dissolve. Whisk very well, then add the cocoa. Whisk until the batter starts to clump together around the whisk.
03 -
Add the egg, egg yolks, vanilla, and salt. Switch to a hand mixer and beat until smooth - about two minutes (you can keep using the whisk if you don’t have a hand mixer, but it will take longer and more effort).
04 -
Add the sourdough discard and flour. Beat until just combined.
05 -
Pour the batter into the prepared pan. Bake until the top is matte and puffed in the center, then collapses - about 40 minutes.