Frosted cake can be made up to two days ahead. Keep in an airtight container. To melt chocolate for icing, place in medium, heat-proof bowl. Sit bowl over a small pan of simmering water. Stir until smooth. Remove and cool.
125g unsalted butter, chopped
150g dark cooking chocolate, chopped
1 1⁄4 cups caster sugar
1⁄3 cup water
1⁄2 cup cocoa powder
3 eggs, lightly beaten
1 1⁄2 cups self-raising flour 1 cup buttermilk
200g unsalted butter, chopped, at room temperature
1 1⁄3 cups icing sugar mixture
200g dark cooking chocolate, melted, cooled
Grease a 22cm round cake pan. Line base and side with baking paper.
2 Combine butter, chocolate, sugar and water in a large saucepan over a low heat. Stir until butter and chocolate are melted and sugar is dissolved. Transfer to a large bowl. Whisk in sifted cocoa until smooth. Stand for 10 minutes.
3 Whisk eggs into chocolate mixture. Add flour and buttermilk, in two batches, whisking until combined. Pour into prepared pan.
4 Cook in a moderately slow oven (160C) for about 1 hour, or until a skewer inserted into the centre comes out clean. Remove. Stand in pan for 15 minutes. Transfer to a wire rack to cool.
5 To make icing, beat butter in a small bowl of an electric mixer until pale. Add sugar, 1⁄2 cup at a time, beating until smooth. Add chocolate. Beat until combined.
6 Cut cake in half horizontally. Sandwich cakes together with half the frosting (about 11⁄4 cups). Spread remaining frosting over the top.
7 Cut into wedges to serve.