Line and grease a 20cm loose bottomed cake tin. Place the chocolate and butter in a saucepan and melt together, stirring constantly. Leave to cool slightly before mixing in the egg yolks, flour, nuts and chocolate pieces. In another bowl, whisk the egg whites to medium peaks. Fold the egg whites into the cake mixture, being very carefully to keep as much air into the mixture as possible. Gently tip the mixture into the prepared cake tin, and level out. Bake at 200`c, in the middle shelf of the oven for about 40 minutes. When the brownie cake is cooked it should be firm in the middle, and may sink slightly in the middle as it cools down. Finally, sift the icing sugar over the top of the cake. The chocolate brownie cake can be served either warm with a scoop of ice-cream, or at room temperature.