Sift flour, sugar, baking powder and salt into mixing bowl of electric mixer. Add shortening. Add 2/3 cup of the milk and the flavoring. Beat until perfectly smooth (about 100 strokes by hand or 2 minutes on medium speed with electric mixer). Add unbeaten egg whites and remaining milk. Beat until well blended or for same period of time or strokes as given above. Pour into 2 greased and floured 8" round cake pans. Bake 25 to 30 minutes @ 350degrees.