In a medium mixing bowl whisk together flour, cornstarch and salt.
In the bowl of an electric stand mixer** mix together 1 cup powdered sugar, butter and lemon zest on low speed until combined. Increase mixer speed to medium-high and whip until slightly pale and fluffy, about 1 - 2 minutes.
Scrape down bowl then mix in egg yolks 1 at a time, blending after each addition. Scrape down bowl.
Add flour mixture to butter mixture and blend until combined (it will seem dry and crumbly at first, keep mixing and it will come together). If dough is sticky at this point you can chill it for 30 minutes but you shouldn't need too if using cool butter.
Scoop dough out 1 Tbsp at a time and transfer to the baking sheet spacing cookies 2-inches apart. Evenly flatten cookies with the heel of your hand to about 1/3-inch thickness*** (spray hand with cooking spray if needed to reduce sticking).
Bake one sheet at a time until cookies appear dry on the surface yet are just slightly soft when pressed, about 8 to 10 minutes.
Let cool on baking sheet for 5 minutes then transfer to wire rack to cool completely. Repeat process with second sheet of cookies.
To make the glaze in a small mixing bowl whisk together 1 cup powdered sugar with the lemon juice. Thin with more lemon juice as needed.
Spoon glaze over cookies and spread into a small thin round. Garnish with a little lemon zest if desired.
Once glaze has set store cookies in a single layer in an airtight container. They'll keep several days at room temperature but can also be refrigerated or frozen for longer shelf life.
Notes
*Use a zester to get the zest of the lemon rather than something like a cheese grater (you'd end up with some of the white pith of a lemon if using a cheese grater and cookies will be bitter).
**Cookies can also be made using a mixing bowl and a hand mixer rather than a stand mixer. I've had success with both options.
***Keep in mind cookies will spread some so don't flatten very much.