Pre-heat oven to 160°C (140° fan-forced). Grease and line the base and sides of a 20cm cake tin with removable base.
Place chocolate and butter in a heatproof bowl over a saucepan of gently simmering water. Stir until melted and smooth. Remove from heat and whisk in the sugar and cocoa. Cool slightly then mix in the egg yolks and flour.
Beat egg whites with an electric mixer until firm peaks have formed. Fold into the chocolate mixture with a large metal spoon until well incorporated. Pour into the prepared pan and bake for 55 minutes. Allow to cool in the pan for 15 minutes, before removing to a cooling rack to cool completely.
For serving, dust with extra cocoa and serve with cream or ice-cream.