In electric mixer beat the egg whites to soft peaks and transfer to another bowl. Mix together flour and salt, set aside. Whisk together yolks with milk and vanilla. In mixer, beat the butter, shortening and sugar until fluffy. On low speed add, afternately, the flour mixture and egg yolk mixture. Beat well. Fold in the egg whites. Put into a greased and floured tube pan. Place in a cold oven and turn to 300 º F. Bake for 45 minutes at that temperature. Raise oven temperature to 325 º F. Bake 45 minutes more.