Preheat oven to 350 degrees F.
In a medium bowl, sift together the flour, baking soda, and salt. Set aside.
In a large mixing bowl, cream the shortening, butter, and sugar until light and fluffy on medium speed; add egg yolks 1 at a time, beating until all the yellow disappears. Add the vanilla. Add the dry ingredients alternately with the buttermilk to the creamed mixture beginning and ending with the flour mixture.
In a small bowl, beat the egg whites on high speed until stiff peaks form. Fold into cake batter.
Fill standard paper-lined muffin cups 3/4 full. Bake for 20 to 25 minutes or until a toothpick inserted in center comes out clean. Let cook in pan for 10 minutes. Transfer cupcakes to wire rack that has been placed over a baking sheet to cool.