Preheat oven to 160Line and grease an 8inch round cake tin.
Cream Butter and Sugar until light and fluffy, add egg and beat well. Then add milk and sifted flour and cacao alternatively. Last add boiling water and pour into prepared tin. Cook in moderate oven for 20-25min or until cooked through.
Meanwhile prepare the icing. Mix the icing sugar, cacao and butter together. Add in one tbsp milk, then 1 tsp of milk until desired consistency is reached.