2 1/2 c. sifted flour
1 tsp. baking soda
1/2 tsp. cream of tartar
1 c. (2 sticks) unsalted butter, softened
1 1/4 c. sugar
3 egg yolks
1 tsp. vanilla

Preheat oven to 350 degrees. Grease cookie sheets. Sift flour, baking soda and cream of tartar. Beat together butter and sugar with electric mixer until fluffy. Beat in egg yolks and vanilla. Gradually add flour mixture in 4 parts, beating well after each addition.

Shape dough into balls using 1 level tablespoon. Place 2 inches apart. Do not flatten. Bake for 12 minutes or until lightly golden around edges. Remove to wire racks to cool.