Beat the butter very well in a bowl on a high speed until lighter in colour.
Add in the icing sugar and combine using a fork (this will save those massive clouds of icing sugar, also your hand/ stand mixer, see this post for more buttercream hints and tips)
Once the icing sugar has been combined with a fork add in your vanilla extract.
Using your mixer, add teaspoon of milk/ water at a time until your buttercream is at the right consistency for your needs.