Preheat the oven to 350°F (175°C) and grease a 9-inch round cake pan.
Melt the dark chocolate and butter together in a double boiler.
Whisk the eggs and sugar together until pale and thick.
Combine the melted chocolate mixture with the egg mixture.
Sift in the cocoa powder and fold until well mixed.
Pour the batter into the prepared cake pan.
Bake for 25 minutes until the edges are set.
Let cool for 10 minutes before inverting onto a plate.
Dust with powdered sugar and serve with fresh berries if desired.