Cream the butter on its own for a few minutes. Add in the sifted icing sugar and mix well.
Finally, add in the vanilla extract and mix at a high speed until nice and creamy.
Make sure to not add any other liquid to the buttercream, as it needs to remain firm.
If the buttercream is too firm, microwave it for a few seconds.
You can also make the buttercream in advance. Simply pop in a tupperware box, press down on the buttercream with a layer of clingfilm to prevent crusting, and keep in the fridge with a lid on. This will keep until the Best Before date on your butter.