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Basic buttercream icing for icing cakes and biscuits 

What you will need:
2-3 Tbsp hot water
1/2 -1 tsp of vanilla essence
Food Colouring (optional)
150g butter (softened)
2 cups Icing Sugar

Method:
Beat the butter until smooth, pale and fluffy. 
Gradually add the Chelsea Icing Sugar, half a cup at a time, 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.

Very Simple 

Note:  When adding food coloring 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