Whisk flour, cornstarch, baking soda, and salt in a medium bowl.
In a large bowl, cream butter and sugar until light and fluffy.
Add egg, lemon juice, lemon zest, and vanilla; beat until combined (it may look curdled).
Gradually mix in dry ingredients until a thick, sticky dough forms.
Cover and chill dough for at least 3 hours (up to 3 days).
Preheat oven to 350°F (177°C); line baking sheets with parchment or silicone mats.
Scoop 1 tbsp dough (about 20g), roll into balls.
Roll balls in granulated sugar (optional), then generously in confectioners’ sugar.
Place 3 inches apart on baking sheet and bake for 12–13 minutes, until edges are set.
If not spreading by minute 9, tap the tray gently on counter, then finish baking.
Cool on tray 5 mins, then transfer to wire rack to cool completely.
Store in an airtight container at room temperature for up to 1 week.