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.
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.
Operating under the strict thermal constraint of a 350 degree Fahrenheit oven requires precise management of the baking sheet to ensure even heat distribution. Aluminum pans conduct heat rapidly, meaning dark non-stick surfaces will brown the bottoms of these confections long before the interior sets and the characteristic fissures form on top. Setting the timer for a strict range of 10 to 11 minutes prevents the interior moisture from evaporating entirely, safeguarding the tender crumb against excessive dryness.
Creaming softened butter with granulated sugar incorporates microscopic air cells that expand under heat, which gives the cookie its initial lift without requiring chemical leaveners to do all the heavy lifting. Beating the fat and sugar thoroughly for several minutes transforms the mixture into a pale, fluffy paste before the liquid components enter the bowl. Adding the egg yolks independently of whites introduces extra lipids from the yolk, resulting in a richer, denser texture that contrasts with standard whole-egg sugar cookies.
Forming the dough into untrimmed walnut-sized spheres rather than flattening them encourages upward expansion and the signature surface cracking as the edges set faster than the center. Leaving two inches of space between each portion on the metal sheet accommodates the lateral spread that occurs during the first few minutes of thermal exposure. When the edges show a faint golden hue while the central fissures remain pale and matte, remove the pans immediately to stop the cooking process.
Altering the primary fats by substituting margarine or shortening changes both the spread rate and the melting point, frequently yielding a flatter product with a greasy mouthfeel due to the absence of milk solids found in real butter. Pastry flour can replace all-purpose flour at a one-to-one volumetric ratio, though the lower protein content removes structural integrity and produces a crumbly texture that struggles to maintain shape during handling.