In large mixing bowl, beat butter until fluffy. Add three cups sugar, 1 tablespoon milk, and vanilla extract.
Beat until combined and fluffy.
Adjust with more sugar or milk, depending on what consistency you're looking for.
Adaptations: swap out the vanilla extract for another flavor extract if you prefer. You can also add the zest of a lemon, lime, grapefruit, or orange. For peppermint, stir in crushed peppermint candies or 1/4 teaspoon peppermint extract.