1.Preheat oven to 180°C/160°C fan-forced. Grease deep 20cm-round cake pan; line with baking paper.
2.Beat butter, extract, sugar, eggs, sifted flour and cocoa,
and the water in large bowl with electric mixer on low speed until ingredients are combined. Increase speed to medium; beat about 3 minutes or until mixture is smooth and paler in colour.
3.Spread mixture into pan; bake about 1 hour. Stand cake 5 minutes; turn, top-side up, onto wire rack to cool.
4.Meanwhile, make chocolate icing. Spread cake with icing.
5.Melt chocolate and butter in small heatproof bowl over small saucepan of simmering water; gradually stir in sifted icing sugar and the hot water, stirring until icing is spreadable.
Chocolate cake can be stored in an airtight container for up to 3 days.