Sugar Cookies

Preheat oven to 300 degrees F. In a large mixing bowl combine butter and shortening. Beat with an electric mixer on medium to high speed for 30 seconds. Add sugar, baking soda, cream of tartar, and salt. Beat until combined, scraping sides of bowl occasionally. Beat in egg yolks and vanilla until combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour.

Shape dough into 1-inch balls. Place balls 2 inches apart a parchment-lined or ungreased cookie sheets.

Bake for 12 to 14 minutes or until edges are firm and bottoms are very light brown. (Do not let edges brown.) Cool on cookie sheets for 2 minutes. Transfer to a wire rack; cool completely.