Preheat oven to 375°F. Grease an 8” round cake pan well and line the bottom with parchment. Grease the parchment and lightly dust bottom and sides with cocoa, shaking out any excess. Set aside.

In a medium bowl, melt chocolate, butter, and espresso in the microwave in 30-second increments, stirring after each, until smooth. Let mixture cool slightly.

In a separate bowl, vigorously whisk the sugar, eggs, vanilla, and salt for 45 seconds, until somewhat thickened and foamy.

Add cocoa powder and whisk until smooth.

Add chocolate mixture and gently fold with a spoon or spatula until combined.

Pour batter into prepared pan, smoothing into an even layer. Bake for 21–25 minutes, until the center is just set but still soft. Don’t overbake!

Cool in the pan on a wire rack for 45 minutes. Run a knife around the edges to loosen cake, then flip it onto the rack, peel off the parchment, and gently flip back over.

If serving immediately, the center will be gooey, soft, and almost molten-fudgy. For a denser cake that’s easier to slice, chill for 1–2 hours before serving.