These classic treats can be made ahead and frozen for holidays and bake sales.
Preheat oven to 300 degrees F. In a large mixing bowl beat butter and shortening with an electric mixer on medium to high speed for 30 seconds. Add sugar, baking soda, cream of tartar, and salt. Beat mixture until combined, scraping sides of bowl occasionally. Beat in egg yolks and vanilla. 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 on ungreased cookie sheets lined with parchment paper.
Bake in the preheated oven for 12 to 14 minutes or until edges are set; do not let edges brown. Cool cookies for 2 minutes on cookie sheet. Transfer cookies to wire racks and let cool.
To make ahead: Layer cookies between wax paper in an airtight container; cover. Store at room temperature for up to 2 days or freeze for up to 3 months.
1 other carb, 1 fat