In a medium bowl, sift together flour, cornstarch, baking powder, and baking soda. Set aside.
In a mixer bowl fitted with the paddle attachment, beat butter and sugar on medium speed until light and fluffy, about 3 minutes. Beat in egg yolks and vanilla extract just until combined. Reduce speed to low. Add the flour mixture and beat just until combined. Do not overmix or the cookies will turn out tough.
When making the dough, if it feels dry, you can add a little bit of cold water or milk a little at a time, just until the dough comes together.
Form the dough into a ball, then flatten slightly to form a disc. Wrap with plastic wrap and refrigerate for 1-2 hours, until firm enough to roll.
Preheat oven to 350℉/177℃. Bake for 7-10 minutes, or until cookies appear golden brown at the edges. Allow cookies to cool in the pan for 10 minutes, then gently transfer to a wire rack to cool completely.