Put the flour, salt, and cornstarch in a medium bowl. Whisk to combine and set aside.
Combine the powdered sugar and butter in the bowl of a stand mixer with a paddle attachment. Mix on medium speed until light and fluffy, about 3 minutes. Add the egg yolks and vanilla. Mix just until incorporated.
On low speed, slowly add the flour mixture. Once it’s all been added, mix until it is just incorporated and no white pockets remain.
Divide the dough in half and roll each piece into a log about 1½ inches [4 cm] thick. Wrap in plastic and refrigerate until firm, at least 2 hours.
While the dough is chilling, adjust the oven rack to the middle,
Once the dough is firm, remove from the refrigerator and peel off the plastic wrap. Slice into ¼-inch- [6-mm-] thick rounds. Place the rounds on the prepared sheet pans, leaving ¾ to 1 inch [2 to 2.5 cm] of space between the cookies.
Once the cookies are completely cool, flip half of them over. Carefully spread