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

In a medium saucepan over low heat, melt dark chocolate and unsalted butter until smooth.

In a large bowl, whisk together granulated sugar and eggs until pale and fluffy, about 3 minutes.

Stir in the melted chocolate mixture and freshly grated orange zest until well combined.

Pour the batter into the prepared cake pan and smooth out any lumps.

Bake for 25–30 minutes until set but still slightly soft in the center. Allow to cool for 10 minutes before transferring to a wire rack. Dust with confectioners' sugar before serving.