Melt the butter over a gentle heat.
Add the brown sugar and milk and stir gently until the sugar has dissolved.
Bring to the boil and boil for 1 minute.
Cool the mixture slightly, then beat in the icing sugar.
Flavour as required.
Use the fudge icing quickly before it becomes too hard to handle.
Variation Ginger: sift 1 teaspoon of ground ginger with the icing sugar before adding to the melted mixture.