Use a paddle attachment to beat the softened butter until light and fluffy.
Add 3 cups of powdered sugar and turn the mixer on the lowest speed (so powdered sugar doesn't go flying everywhere) until the powdered sugar starts to become absorbed into the butter. Turn the mixer up to medium speed and add the salt, vanilla and 3 tbsp of heavy cream and beat for 3 minutes.
If your frosting needs a thicker consistency add more powdered sugar. If it needs to be thinned out, add a little more heavy cream.