Preheat your oven to 350°F (175°C).
In your mixing bowl, add the very soft unsalted butter, granulated sugar, light brown sugar, and the egg yolk. Mix until smooth and creamy.
Stir in the vanilla extract.
Sprinkle the all-purpose flour, baking soda, cornstarch, and kosher salt over the wet ingredients. Fold until just combined.
Fold in the semi-sweet chocolate chips.
Form the dough into a large ball and place it on the prepared baking sheet. Flatten into a thick disk.
Sprinkle a few more chocolate chips on top before baking.
Bake for about 10-12 minutes, until the edges are lightly golden.
Let the cookie cool on the baking sheet for a few minutes before transferring it to a wire rack.
