Preheat the oven to 350?F (177?C). Line a 9-inch square baking pan with parchment paper.

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

In a medium bowl, whisk the melted butter, brown sugar, and granulated sugar together until no lumps remain. Add the egg, egg yolk, and vanilla extract. The mixture will be thick.

Pour the wet ingredients into the dry ingredients and mix until combined. Fold in the chocolate chips.

Transfer the dough to the prepared baking pan and press into an even layer. Lightly sprinkle the top with flaky sea salt if desired.

Bake for 30-34 minutes or until lightly browned on the sides. Use a toothpick to test for doneness.

Allow the bars to cool in the pan on a wire rack for at least an hour before lifting them out to cut into squares.

Cover and store leftover bars at room temperature for up to 1 week.