1/2 cup butter, softened
1/2 cup shortening
2 cups sugar (I like to use baking sugar for a more delicate texture)
1 tsp. baking soda
1 tsp. cream of tartar
1/8 tsp. salt
3 egg yolks
1/2 tsp. vanilla
1 3/4 cup flour

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 until combined, scraping sides of bowl occasionally. Beat in egg yolks and vanilla until combined. Beat in as much flour as you can, stir in remaining flour by hand if needed.
Shape dough into 1 inch balls; place 2 inches apart on an ungreased cookie sheet. Bake in a 300-degree oven about 15 minutes or until sides are set (do not let edges brown). Cool on a cookie sheet for 1 minute, transfer to a wire rack and let cool.