Using a stand mixer with the paddle attachment, or a hand mixer, add the softened butter to a large mixing bowl. Cream the butter for roughly 30 seconds.
Add the powdered sugar one cup at a time, mixing in between each addition. I usually turn off my stand mixer when adding the powdered sugar so it doesn't go flying everywhere.
After adding about two cups of sugar, add 2 tablespoons of heavy cream/milk. I like to do this in between additions so the mixture doesn't dry out too much.
Continue adding the powdered sugar one cup at a time, and the heavy cream/milk 1 tablespoon at a time, mixing in between each addition.
Add the 1/4 teaspoon salt and 2 teaspoons vanilla, mix to combine.
The frosting shouldn't be runny. You should be able to dip your finger in the frosting and not have it run off. It should seem nice and fluffy.
Remember, add the sugar slowly. You can always add more but you can't take any away.