Preheat oven to 350°F. Line two baking sheets with baking parchment, silicone mat, or lightly grease.
Cream butter and sugar.
Beat in egg yolks and vanilla.
Add flour, and either the baking soda and cream of tartar, or baking powder. Stir until mixed.
Form dough into 1-inch diameter balls and place 2 inches apart. Do not flatten.
Bake 10 to 11 minutes, until tops are cracked and just turning color.