Grease and line the base of a 20 cm/8inch springform or loose based cake tin. Melt the chocolate in a bowl over hot water (do not boil chocolate),then stir in the butter.
Separate egg yolks from white, set aside the whites. Whisk the egg yolks and sugar together until pale in colour. Gently and gradually beat in the chocolate mixture, then stir in the flour.
In a separate bowl whisk egg whites until stiff.Using a large metal spoon fold into the chocolate mixture. Turn into prepared tin and sprinkle walnuts over the top.
Bake in preheated oven 180C/350F/Gas 4 gor 35-40 minutes until risen and only just firm in the centre. Leave to cool slightly, then serve warm, dusted with icing sugar and cocoa powder.
This is something between a pudding and a cake