Line the base of a 20-23cm (8-9 inch) square cake tin. Pre-heat the oven to gas mark 4, 180ºC.

Melt the dark chocolate and butter in a small bowl over a pan of hot water. Allow to cool. Beat the eggs and sugar in an electric food mixer until thick, pale and mousse like. This may take 3-4 minutes.

Fold the cooled chocolate mixture into the egg mixture. Sift the flour and cocoa over the top and gently fold in.  Stir in the chopped chocolate very carefully and pour into the prepared tin. Bake for 35 minutes or until the mixture no longer wobbles when the tin is gently shaken. The top should be dark and shiny.

Cool in the tin for 1 hour. When cold, turn out onto a clean tea towel to preserve the shiny top then invert and cut into triangles. Store wrapped in foil in the refrigerator.

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