Preheat your oven to 375°F (190°C).
Grease an 8-inch round cake pan and line the bottom with parchment paper.
Melt the dark chocolate and butter together in a double boiler or microwave.
Stir until smooth and allow to cool slightly.
In a separate bowl, whisk the sugar and eggs until light and fluffy.
Add the melted chocolate mixture and vanilla extract; mix well.
Sift in the cocoa powder and salt, folding gently until just combined.
Pour the batter into the prepared cake pan.
Bake for 25 minutes, or until the center is set but still soft.
Allow the cake to cool in the pan for 10 minutes, then invert onto a plate.