On medium speed of an electric mixer, beat about 3-5 minutes or until smooth and creamy.
Gradually add the remaining sugar, 1 cup at a time, beating well (about 2 minutes) after each addition, until the icing is thick enough for good spreading consistency (you may not need all of the sugar).
If desired, add a few drops of food coloring and mix thoroughly.
Use and store the icing at room temperature because icing will set if chilled.
The icing can be stored in an airtight container for up to 3 days.
**If making a 3-layer cake, use the following proportions: 1 1/2 cups unsalted butter, 8-10 cups confectioner's sugar, 3/4 cup milk, and 1 tablespoon vanilla extract.