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 eggs, one at a time, beating well after each addition. Beat in vanilla. On low speed, alternately add flour mixture, about 1/3 of mixture at a time, and milk, about 1/2 at a time, beating just until blended. Stir in butterscotch chips. Divide batter evenly among muffin cups, filling each with about 3 tablespoons batter or until 2/3 to 3/4 full.