In a medium bowl, cream together the butter, shortening, and sugar. Beat in the egg yolks one at a time, then stir in the vanilla. Combine the flour, baking soda, and salt; stir into the creamed mixture. Roll dough into walnut size-balls and place 2 inches apart onto an unprepared cookie sheet.
Bake for 20 minutes in the preheated oven. Cool on wire racks.
