Grease and flour 10-inch tube pan. Sift together (three times) flour, baking powder and salt; set aside. Cream butter and sugar until fluffy. Add egg yolks and vanilla and mix well. Add flour mixture alternately with milk, beating after each addition until smooth. In another bowl mix egg whites in high speed until frothy. Finally fold the egg whites in cake mixture. Pour into greased and floured pan. Bake at 325 °F. for 45 to 50 minutes or until toothpick insert in center comes out clean.