It is a slight variant on the recipe in Bee Nilson's "The Penguin Cookery Book" which everyone should look for in their local charity shop as I have never seen it anywhere else and it is amazing.
Ingredients:
250g icing sugar
35g unsalted butter
3 tsp water or milk
1 tsp vanilla extract (or whatever flavoring you want)
Method:
Mix butter on it's own in order to soften it
Then beat in the icing sugar
Add the liquid and extract in order to create the desired consistency.
I find that in order to create a smooth butter cream icing the best thing to do is to just keep mixing for as long as possible so that any lumps however small are no longer there.
