Mix butter on medium speed in an electric mixer, until smooth.

Add one cup of powdered sugar at a time, on low speed, until fully incorporated. Start with 3 cups and only add the last cup if needed to thicken at the end.

Add salt and vanilla and mix on medium. Add 2 tablespoons of milk and mix for 3 minutes.

Check consistency and add more sugar or milk depending on spreadability. Add powdered sugar if too loose and add more milk if too stiff.