Beat the butter and icing sugar until light, fluffy and pale yellow.
Place the milk and caster sugar ina saucepan and bring to the boil.
Blend the extra milk with the custard powder and eggs.
Pour the hot milk over the custard mixture, stirring all the time. Return to the saucepan and cook over a medium heat until thickened. Do not allow to boil. Remove from the heat and stir in melted chocolate.
Pour into a shallow dish, sprinkle with extra caster sugar to prevent a skin from forming and chill quickly.
When the custard is cool, beat into the creamed butter and icing sugar a spoonful at a time.
Beat for 10 minutes and then ice your cake.
This recipe makes enough to cover one 23cm cake
