1.Preheat oven to 160 °C. Grease 2 deep 20cm round cake pans and line bases with paper.
2.Cream butter, sifted cocoa and sugar in small bowl with electric mixer until light and fluffy, then beat in egg yolks. Transfer mixture to large bowl, fold in sifted flours and water.
3.In a small bowl, beat egg whites in until soft peaks form. Fold lightly into chocolate mixture.
4.Pour into prepared pans and bake for about 1¼ hours. Turn on to wire rack to cool. Join cakes with whipped cream and dust with icing sugar if desired.