Heat oven to 180C/fan 160C/gas 4. Butter a 20-25cm cake tin and line with greaseproof paper.
Place 175g/6oz of the chocolate, plus the butter and sugar in a heavy-based pan and heat gently until melted, stirring occasionally. Leave to cool.
Whisk the egg yolks into the chocolate mixture, then add the flour, nuts and the remaining chocolate.
Whisk the egg whites until they form soft peaks, then gently, but thoroughly, fold into the chocolate mixture.
Pour into the prepared tin and bake in the centre of the oven for about 35-40 mins until crusty on top. Leave to cool, then run a knife around the sides and remove from the tin. Dust with icing sugar and serve warm with custard or ice cream or cold with cream.