Preheat your oven to 350°F (175°C). Grease an 8-inch round cake pan and line the bottom with parchment paper.

Melt the dark chocolate and unsalted butter together in a heatproof bowl set over simmering water until smooth.

In another bowl, whisk the eggs and granulated sugar until pale and thickened, about five minutes.

Combine the melted chocolate mixture into the egg-sugar mix while whisking continuously. Stir in the orange zest.

Sift in cocoa powder and salt, gently folding until no dry pockets remain.

Pour the batter into the prepared pan and bake for 25-30 minutes until set but still slightly jiggly in the center. Let cool before removing from the pan.