Preheat oven to 325°F (165°C). Grease a 9x13 inch baking pan or line with parchment paper, leaving an overhang on the sides for easy removal.

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

In a large bowl, whisk the melted and cooled butter with the brown sugar and granulated sugar until no lumps remain.

Whisk in the egg, egg yolk, and vanilla extract until the mixture is smooth and slightly glossy.

Using a rubber spatula, fold the dry ingredients into the wet ingredients until just combined. Do not overmix.

Fold in 1 1/4 cups of the chocolate chips until evenly distributed.

Transfer the dough to the prepared baking pan and press it into an even layer using a spatula or damp fingers.

Sprinkle the remaining 1/4 cup of chocolate chips over the top, pressing them slightly into the dough.

Bake for 23 to 27 minutes, or until the edges are very lightly browned and the top appears set. The center should still look slightly soft; do not overbake.

Place the pan on a wire rack and allow the bars to cool completely in the pan to room temperature before slicing into 24 squares.