½-1 tsp of vanilla essence (or flavour of your choice)
Food Colouring (optional)
Method
Beat the Butter until smooth, pale and fluffy. Gradually add the Icing Sugar, half a cup at a time.
Add the vanilla essence (or other flavoured essence and/or colouring of your choice), with enough of the hot water to make a creamy mixture, beating between each addition until fluffy.
Pipe onto cooled cupcakes or spread over a cooled cake.
Tips
When adding food colouring add it a few drops at a time and make sure you mix it in otherwise you might end up with a brighter result than you desire.