Beat butter in large bowl with electric mixer on medium speed until light and fluffy. Add Root Beer Concentrate; mix well.
Gradually add confectioners' sugar, beating until well blended after each addition and frequently scraping sides and bottom of bowl.
Add milk; beat until light and fluffy. If frosting is too thick to spread, gradually beat in additional milk. Store in refrigerator up to 2 weeks. Rewhip before using.