1.Grease a deep 23cm round cake pan, line base with paper; grease paper.
2.Combine butter, essence, sugar, eggs, sifted flour and cocoa and water in large bowl, beat on low speed with electric mixer until ingredients are combined.
3.Increase speed to medium, beat for about 3 minutes or until mixture is smooth and changed in colour. Spread into prepared pan.
4.Bake at 180°C/355°F for about 1½ hours. Stand 5 minutes before turning on to wire rack to cool.
5.Meanwhile make chocolate icing: melt chocolate and butter in bowl over hot water, gradually stir in sifted icing sugar, then stir in enough hot water to mix to a spreadable consistency.
6.Spread cold cake with icing.