Line a large rimmed baking sheet with parchment paper. In a medium bowl, whisk the all-purpose flour, cornstarch, baking soda, and fine salt for about 30 seconds, breaking up every visible lump of cornstarch. Set this mixture aside. Keep the baking sheet at room temperature for now, and confirm there is enough refrigerator space for it before mixing the dough.
Place the 60 g granulated sugar and lemon zest in a large mixing bowl. Rub them together with your fingertips for about 30 seconds, until the sugar feels slightly damp and smells strongly of lemon. Add the softened butter and brown sugar. Beat with an electric mixer on medium speed for 2 minutes, scraping the bowl once, until creamy, cohesive, and slightly lighter but not fluffy or warm.
Add the room-temperature egg yolk and beat on medium-low speed for 30 seconds, stopping to scrape the bottom and sides of the bowl. Add the measured lemon juice and vanilla, then mix for another 20 to 30 seconds. The mixture should look smooth and creamy, although slight curdling from the acidic juice is harmless. Do not continue beating once the liquids are evenly incorporated.
Add the dry ingredients and mix on low speed just until most flour disappears, about 20 seconds. Finish folding with a flexible spatula, scraping the bowl until no dry pockets remain; the dough should be soft but scoopable. Divide it into eight portions of about 48 g each and roll them gently into balls. Arrange them on the prepared sheet, cover without pressing the covering onto the dough, and refrigerate for 45 minutes.
During the final 20 minutes of chilling, position a rack in the center of the oven and heat it to 350°F or 175°C. Put the remaining 20 g granulated sugar in a shallow bowl. Roll each cold dough ball in the sugar, covering the surface evenly, then return it to the lined sheet. Space the portions at least 3 inches apart and leave them rounded rather than flattening them.
Bake the cookies on the center rack for 10 to 12 minutes, rotating the pan after 7 minutes if the oven browns unevenly. They are done when the edges are set and faintly golden, the tops have delicate cracks, and the centers remain pale and slightly puffed without glossy raw dough. Cool on the pan for 10 minutes, then transfer carefully to a wire rack and cool for at least 5 minutes before serving.