Preheat the oven to 350 degrees. Spray a 9-inch cake pan with non-stick spray, line it with parchment, then spray again.
In a double boiler, melt the butter and chocolate, stirring every so often, and set aside to cool for a few minutes.
Whisk in the sugar and the salt, then whisk in the eggs one at a time. It’ll get harder to whisk with each addition.
Sift in the cocoa powder and the espresso powder and fold together with a rubber spatula until just incorporated.
Pour into the prepared pan and bake for 35 to 40 minutes or until a cake tester comes out mostly clean (it’s okay if there are crumbs). Cool for ten minutes, flip out on to a plate to remove the parchment, then flip on to a cooling rack. Cool completely. Place on a cake stand and dust with powdered sugar. Serve as-is or, if you care about the people in your life, whip some cream by hand to put on top.