01 - Preheat oven to 350°F. Grease an 8-inch round cake pan and line the bottom with parchment paper.
02 - Combine bittersweet chocolate and unsalted butter in a heatproof bowl and melt together, stirring until smooth.
03 - In a large bowl, vigorously whisk eggs and granulated sugar until mixture turns pale and fluffy.
04 - Pour the melted chocolate mixture into the egg mixture. Add espresso powder, vanilla extract, and a pinch of salt. Fold until fully combined.
05 - Transfer batter into prepared pan. Bake for 25 to 30 minutes, or until just set in the center.
06 - Allow cake to cool completely in the pan on a wire rack. Once cool, remove from pan and dust with cocoa powder before slicing.