In a stand mixer (paddle attachment) or with a hand mixer, beat the butter at medium high speed until light and creamy--about 3 to 4 minutes. Add the rest of the ingredients (start with 1/2 c. milk) with the mixer on low, then move the speed up to medium high for two minutes, scraping down the sides after each minute. (If it's too thick, you can add a little more milk at a time if needed. It should be pretty thick, but spreadable.)
Notes
Many recommend that frosting should be made with unsalted butter, but I regularly use salted butter, if that's what I have on hand. If you do, just reduce the salt a little bit in the recipe.