Preheat oven to 180°C. Grease a 22cm (top) fluted ring bundt pan. Beat butter and caster sugar until pale and creamy. Beat in eggs and vanilla essence.
Fold in self-raising flour and milk. Divide mixture into 2 portions. Stir in 1 tablespoon cocoa powder to one portion. Alternately spoon the mixtures into the pan.
Bake for 40-45 minutes. Set aside for 15 minutes before transferring to a wire rack to cool. Combine icing sugar, 2 tablespoons cocoa powder and 20g chopped butter. Stir in boiling water. Pour over the cake to serve.