Hazelnut bitter chocolate cake

375g/12oz unsalted butter
380g/12oz bitter chocolate
300g/10oz caster sugar
9 eggs separated
75g/3oz plain flour
60g/21/2oz hazelnuts
Heat the oven to 160C/325F/Gas3. Brush a 26cm (10in) cake tin with melted butter. Place the butter and chocolate in a bowl over a pan of hot water until it melts. Leave it to cool slightly. Mix three-quarters of the sugar with the yolks until well blended and fold into the chocolate mixture. Stir in the flour and hazelnuts. Whisk the whites with the remaining sugar until soft peaks are formed and fold this carefully, but thoroughly, into the chocolate mixture. Pour into the cake tin and bake for around 40-50 minutes.