In large bowl, beat butter with electric mixer on medium speed 30 seconds. Gradually add granulated sugar, about 1/4 cup at a time, beating well after each addition and scraping bowl occasionally. Beat 2 minutes longer. Add whole eggs, one at a time, beating well after each addition. Beat in 1 teaspoon vanilla. On low speed, alternately add flour mixture, about one-third a time, and 2/3 cup root beer, about half at a time, beating just until blended.