Preheat the oven 180C/350F/Gas 4. Butter a 20cm/8in springform cake tin with a removable base.
Melt the chocolate and butter in a bowl over simmering water.
Whisk the egg whites to stiff peaks and in another bowl whisk the yolks with the sugar until pale.
Add the chocolate mixture to the egg yolks and sugar and fold in the egg whites.
Pour into the cake tin and bake for about 20 minutes. Cool and dust with icing sugar and cocoa powder.