Mix Dry Ingredients: In a medium bowl, whisk together flour, cornstarch, baking soda, and salt. Set aside.
Cream Butter & Sugar: In a large mixing bowl, beat butter and sugar together until light and fluffy.
Add Wet Ingredients: Mix in egg, lemon juice, lemon zest, and vanilla extract until combined.
Combine & Chill: Gradually mix in the dry ingredients until a soft dough forms. Cover and chill in the refrigerator for at least 3 hours.
Preheat & Prep: Preheat oven to 350°F (177°C). Line baking sheets with parchment paper.
Roll & Coat: Scoop 1 tbsp-sized dough balls, roll first in granulated sugar (optional), then in confectioners’ sugar.
Bake: Arrange on baking sheets 3 inches apart and bake for 12-13 minutes or until edges are set.
Cool: Let cookies cool for 5 minutes on the baking sheet before transferring to a wire rack.