Lightly grease 9-inch (22cm) round cake pan with butter. Line base with parchment paper.
Sift together flour, cocoa, baking powder, baking soda and salt into a large bowl. Whisk in sugar, then add eggs, milk, oil and vanilla. Whisk well to combine until free from lumps.
Pour boiling water into the cake mixture. Please note that the cake batter will be thin in consistency.
Pour batter into cake pan and bake for 20-30 minutes or until a wooden skewer inserted into the centre comes out clean.
Let cool completely before frosting
CHOCOLATE ICING
Combine all the icing ingredients in a sace pan, except for the butter. MIx well until free from lumps.
Cook on medium heat mixing continuously until thick then remove the pan from heat.