Preheat oven to 350 degrees F (180 degrees C). Lightly grease 2 cookie sheets.
Cream together sugar and butter. Beat in egg yolks and vanilla.
Add flour, baking soda, and cream of tartar. Stir.
Form dough into walnut size balls and place 2 inches apart on cookie sheet. Don't flatten. Bake 10 to 11 minutes, until tops are cracked and just turning color.
