Preheat the oven to 350°F (177°C). Line a 9-inch square baking pan with parchment paper, leaving enough paper around the sides to easily lift the bars out of the pan. Set aside.

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 brown sugar lumps remain. Add the egg, extra egg yolk, and vanilla extract. The mixture will be thick. Pour the wet ingredients into the dry ingredients and mix together until combined. The dough will be soft and slick, yet thick. Fold in the chocolate chips. The chips may not stick to the dough because of the melted butter, but do your best to combine them.

Transfer dough to the prepared baking pan and press into an even layer. Lightly sprinkle the top with flaky sea salt, if desired. Also, I like to press a few more chocolate chips on top of the dough before baking; this is just for looks.

Bake for 30-34 minutes or until lightly browned on the sides. Err on the side of under-baking and do NOT over-bake, or the cookie bars will taste a little dry and cakey. Use a toothpick to test for doneness; if it comes out mostly clean from the center of the pan, the bars are done.

Allow the bars to cool in the pan set on a wire rack for at least an hour. Once relatively cool, lift the bars out of the pan using the parchment and cut into squares.

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