Preheat the oven to 350°F (175°C).
Melt the butter and let it cool for approximately 5 minutes. It should remain liquid but should not be hot.
In a medium bowl, whisk together the all-purpose flour, baking soda, cornstarch, and salt. Set aside.
In a large mixing bowl, whisk the slightly cooled melted butter, light brown sugar, and granulated sugar until smooth and well combined.Add the whole egg, egg yolk, and vanilla extract. Whisk until the mixture is smooth and glossy.
Add the dry ingredients to the wet ingredients.
Mix just until a thick cookie dough forms and no dry streaks of flour remain. Do not overmix.Fold in the chocolate chips until evenly distributed throughout the dough.
Cover the bowl tightly with plastic wrap and refrigerate the cookie dough for 1 hour.
Line a 9-inch square baking pan (23 x 23 cm) with parchment paper, leaving enough overhang on the sides to lift out the baked bars.
Transfer the chilled dough to the prepared pan and press it into a smooth, even layer. Sprinkle a few additional chocolate chips over the top.
Bake for 30-40 minutes, or until the edges are golden and the center is set but still looks slightly soft. Avoid overbaking because the bars will continue to firm up while cooling.
Remove the pan from the oven and allow the cookie bars to cool in the pan for at least 30 minutes.
Lift the bars from the pan using the parchment-paper overhang. Sprinkle lightly with flaky sea salt, if desired, and cut into 12 equal squares.
