Preheat oven to 180C. Grease and line a 25cm round cake tin.
Mix all cake ingredients using an electric mixer. Mix on medium speed until smooth.
Pour cake batter into lined cake tin and bake for an hour or until skewer comes out clean. Do not open oven while cooking.
Allow to cool before turning out on plate.
TO MAKE FROSTING:Melt chocolate in a medium heatproof bowl over a saucepan of simmering water until smooth. Cool slightly. Cream butter, icing sugar and vanilla essence together. Add egg yolk and melted chocolate and beat until fluffy. Decorate cake.
For a decadent, but simple dessert, serve with vanilla bean ice-cream and fresh berries.