Heat the oven to 180ºC. Grease a 25cm cake tin. Sprinkle a little bread crumbs into the tin and shake to coat the sides.
Put the chocolate and butter together in a bowl over a pan of hot water (not boiling water). Stir until smooth. Cool slightly.
Put the egg yolks and sugar in a bowl and whisk until pale and thick. Whisk in the chocolate mixture. Fold in the flour, stirring well to combine.
Whisk the egg whites to soft peaks. Fold a third of the egg white mixture into the chocolate mixture to loosen. Then fold in the remaining egg whites. Pour into the prepared cake tin.
Bake for 30 minutes until set and done. Let it cool and then place in wire rack. Sprinkle with icing sugar or cocoa powder.
