In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt. Set aside.

In a large mixing bowl, beat the softened butter and granulated sugar on medium-high speed until light and creamy (around 3 minutes). Use a hand or stand mixer fitted with the paddle attachment.

Add the egg, lemon juice, lemon zest, and vanilla extract to the butter-sugar mixture. Mix on high until fully combined. (Don’t worry if the mixture appears a bit curdled—that’s normal.)

Gradually add the dry ingredients to the wet mixture. Beat on low speed until the dough is thick, creamy, and sticky.

Cover the cookie dough tightly and refrigerate for at least 3 hours (up to 3 days). This step is essential to achieving the right texture.

When ready to bake, preheat your oven to 350°F (177°C). Line baking sheets with parchment paper or silicone mats.

Scoop the dough into tablespoon-sized balls (roughly 20g each). To prevent sugar absorption, roll each ball first in granulated sugar (optional) and then generously in confectioners' sugar.

Place the cookie balls 3 inches apart on the lined baking sheets. Bake for 12–13 minutes, or until the edges are set and centers remain soft.

Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. They’ll slightly deflate as they cool.