Instructions

for the cookies

Combine flour, cornstarch, salt, baking powder, and baking soda in medium bowl; set aside. Using stand mixer fitted with paddle, beat butter and sugar on medium-high speed until pale and fluffy, about 3 minutes. Add lemon juice, egg yolk, and lemon extract and beat until combined.
Reduce speed to low and add flour mixture in 3 additions until just combined, scraping down bowl as needed. Turn out dough onto counter and divide in half. Form each half into 5-inch disk, wrap disks tightly in plastic wrap, and refrigerate for at least 1 hour or up to 24 hours.
Adjust oven rack to middle position and heat oven to 325 degrees. Line 2 baking sheets with parchment paper. Remove 1 dough disk from refrigerator and knead dough for 3 to 5 turns to make more pliable. Roll into 10-inch circle, about 1/4 inch thick, on lightly floured counter.
Using 2-inch round cookie cutter, cut 14 to 15 rounds from dough. Reroll scraps up to 2 times to similar thickness and cut out remaining 5 to 6 rounds to yield 20 cookies. Space cookies 1 inch apart on 1 prepared sheet.
Bake cookies, 1 sheet at a time, until edges are lightly browned, 12 to 14 minutes, rotating sheet halfway through baking. Let cookies cool on sheet for 5 minutes, then transfer to wire rack to cool completely. Repeat with second dough disk and second baking sheet.

for the glaze

Whisk sugar, lemon zest and juice, and salt together in bowl. Working with 1 cookie at a time, dip top of cookie into glaze, then drag top lightly against rim of bowl to remove excess glaze. Repeat. (If glaze thickens as it sits, add extra lemon juice as needed to maintain proper consistency.) Let glazed cookies set fully before serving, about 15 minutes.