Whisk flour, cornstarch, baking soda, baking powder, and salt. Line baking sheets with parchment and set the coating sugar aside.
Rub lemon zest into the dough sugar until fragrant. Beat with butter for 3 minutes until creamy and slightly lighter.
Beat in egg, yolk, lemon juice, and vanilla. Scrape the bowl so the citrus flavor is evenly mixed into the base.
Add dry ingredients on low speed until combined. Cover and chill for 45 minutes so the dough is easy to portion.
Heat the oven to 350°F (175°C). Roll into 24 balls and coat each lightly in the separate sugar.
Arrange 2 inches apart on a cool sheet. Bake 10–12 minutes, until edges look set and the tops lose their wet shine.
Leave on the baking sheet for 5 minutes before transferring to a rack. The centers should remain tender rather than crisp.
Cool completely and store in a closed container. Use cooled baking sheets for remaining portions to keep spreading consistent.
