Beat the softened butter using either a hand mixer or free-standing electric mixer until smooth. Gradually add the icing sugar in 3 separate additions and beat fully between each. Add the vanilla and milk and beat to fully combine. On full speed beat the buttercream for at least 5 minutes until fluffy, smooth and lighten in colour.
Spread and fill a 20cm cake or fill a large pipping bag and pipe over cupcakes.