Preheat oven to 350° F. Spray a 9x9-inch square baking pan with cooking spray, then line with parchment paper.
In a medium bowl whisk together the flour, cornstarch, baking soda, and salt. Set aside.
2 cups (240g) all-purpose flour*, 2 teaspoons cornstarch, 1 teaspoon baking soda, ½ teaspoon salt
Use an electric mixer* to beat together the melted butter, brown sugar and granulated sugar until well combined. Add the egg, egg yolk and vanilla and mix on medium-low speed until well combined.
¾ cup (170g) unsalted butter, 1 cup (213g) packed light brown sugar, ¼ cup (50g) granulated sugar, 1 large egg, 1 large egg yolk, 2 teaspoons vanilla extract
With the mixer on low speed, mix in the dry ingredients until just combined, scraping down the sides of the bowl as needed. Stir in the chocolate chips.
1 cup semi-sweet or bittersweet chocolate chips
Press the mixture into an even layer in the prepared baking pan. Add a few extra chocolate chips on top of the dough, if desired.
Bake for 24-28 minutes (I baked mine for 26 minutes in a metal baking pan), until a tester inserted into the center comes out clean. Be careful not to over-bake, since this will dry out the cookie bars.
Optional: Sprinkle flaky sea salt over the top of the cookie bars as soon as the come out of the oven.
Let cool in the pan for at least 30 minutes. When you're ready to cut the bars, use the parchment paper to lift them out of the pan, then slice into bars with a sharp knife.