Heat oven to 350°F. Grease 8- or 9-inch round pan with shortening; lightly flour. In small bowl, mix flour, baking powder and salt. In medium bowl, beat 1/4 cup butter with electric mixer on medium speed 30 seconds. Beat in granulated sugar, 1/4 cup at a time, beating well after each addition. Beat 2 minutes longer, scraping bowl occasionally. Add egg whites, one at a time, beating well after each addition. Beat in vanilla. On low speed, alternately add flour mixture, one-third at a time, and milk, about one-half at a time, beating just until blended. Spread in pan.