Beat butter in a stand mixer (paddle attachment) on medium for 5 minutes — until pale and fluffy.

Add half the sifted icing sugar. Beat on low until incorporated, then medium for 2 minutes.

Add remaining icing sugar, vanilla, salt and 2 tbsp milk. Beat on medium 5 minutes more.

If too thick, add another tbsp milk at a time. If too thin, add more icing sugar.

Use immediately, or store at room temp 24 hours, or fridge 1 week (re-beat before using).