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.
Ingredients
Method
Grease a 22cm round cake pan. Line base and side with baking paper.
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.
Whisk eggs into chocolate mixture. Add flour and buttermilk, in two batches, whisking until combined. Pour into prepared pan.
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.
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.
Cut cake in half horizontally. Sandwich cakes together with half the frosting (about 11⁄4 cups). Spread remaining frosting over the top.
Cut into wedges to serve.