Beat butter and half of the powdered sugar until well-combined (starting out the mixer low and increasing to medium-high). Slowly add the remaining powdered sugar (while beating), followed by vanilla extract, salt, and milk. Continue to add more milk, a little bit at a time, until you get a creamy spreadable frosting. If desired, you can add food coloring at this point.
Turn the mixer to medium-high and beat well. Depending on how thick you want the frosting, you can easily adjust the consistency by adding more powdered sugar (to thicken) or more milk (to thin).