Cut the stork margarine into small pieces and beat in a bowl until soft and creamy.
Sieve in half of the icing sugar and beat until light and creamy with just enough milk to make a thick mixture (be careful adding the milk as it's easy to use too much).
Add the remaining icing sugar and (if using) the peppermint flavouring and beat until light and creamy. If it is too thick add a smidgeon more milk.
If making chocolate icing, mix in the cocoa (smidgeon of milk if needed as previously noted).
Spread over the bottom half of the cake and replace the top.