In large bowl, beat granulated sugar and 1/4 butter with electric mixer on medium speed about 2 minutes or until light and fluffy, scraping bowl occasionally. Beat in sour cream. Beat in egg whites, one at a time, beating well after each addition. Beat in 1 teaspoon vanilla. On low speed, alternately add flour mixture, about 1/3 at a time, and milk, about 1/2 at a time, beating just until blended. Pour into pan.