Preheat your oven to 350°F (175°C) and grease an 8-inch round cake pan.

Melt the dark chocolate and butter together in a heatproof bowl over simmering water.

Stir until smooth, then remove from heat and let cool slightly.

In a separate bowl, whisk together the sugar and salt.

Add the eggs one at a time, whisking well after each addition.

Stir in the melted chocolate mixture until fully combined.

Add the vanilla extract and mix well.

Fold in the cocoa powder until no lumps remain.

Pour the batter into the prepared cake pan and smooth the top.

Bake for 25 minutes or until the edges are set but the center is still soft. Let cool before serving.