- Preheat the oven to 180°C, then rub butter all over the inside of a 9 inch cake pan. Line the bottom of the pan with parchment. (If you don’t have parchment, butter the inside of the cake pan very generously to prevent sticking.)
- In a small sauce pan, melt the butter over medium heat until it is very hot but not bubbling. Add the ground coffee, stir in, and remove from heat. Let sit for five minutes.
- Strain the butter through a very fine mesh sieve into a large mixing bowl. Press down on the butter soaked coffee to extract as much butter as possible. Once it is strained, you’ll notice that the melted butter is brown and has very tiny particles of coffee in it.
- Add the sugar, vanilla, salt, and eggs plus yolk to the butter, then whisk thoroughly to combine. Add the cocoa powder and flour, then stir until just combined. The batter will be very thick.
- Scrape the batter into the prepared pan and smooth the top. Bake for 20 to 25 minutes or until a toothpick inserted in the middle comes out with moist crumbs but no wet batter. Let cool to room temperature on a rack, then slice and serve.