Method
1. Grease a 23cm square cake pan. Line base and sides with baking paper.
2. Beat butter, sugar and vanilla in a small bowl with an electric mixer until light and fluffy. Add eggs, one at a time, beating well after each addition. Transfer mixture to a large bowl.
3. Stir in combined sifted flour and cocoa and buttermilk, in two batches, until well combined. Spread mixture into prepared pan.
4. Cook in a moderate oven (180C) for about 55 minutes or until cooked when tested. Stand cake in pan for 5 minutes before turning onto a wire rack to cool.
5. To make frosting, beat butter in a small bowl with an electric mixer until pale in colour. Gradually beat in icing sugar mixture until fluffy. Stir boiling water into sifted cocoa in a small bowl to form a smooth paste. Add to frosting mixture and beat until well combined.
6. Spread frosting over top and sides of cake. Decorate sides with grated chocolate.

Hint. Cake can be made up to 3 days ahead or freeze frosted cake for up to 2 months. Decorate with chocolate before serving.