Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper or silicone baking mats.

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

In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter, brown sugar, and granulated sugar on medium speed until light and creamy, about 2 to 3 minutes. Add the egg and vanilla and mix until just combined.

Add the flour mixture in two additions, mixing on low after each until just incorporated. Stop mixing as soon as no flour streaks remain.

Use a 2-tablespoon cookie scoop (size #40) to portion the dough onto prepared sheet pans, 8 cookies per pan. Bake one tray at a time on the center rack for 8 to 10 minutes. Cookies should look slightly underdone when removed from the oven. They will finish baking on the warm pan.

Let cookies rest on the pan for a few minutes before transferring to a wire rack. Once fully cooled, store in an airtight container at room temperature for up to 5 days.