In a medium bowl, whisk together the all-purpose flour, baking soda, cornstarch, and salt until evenly distributed.

In a large bowl, whisk the melted butter, brown sugar, and granulated sugar together until no sugar lumps remain and the mixture is smooth.

Add the room-temperature egg, egg yolk, and vanilla extract. Whisk vigorously until the mixture is well-combined and slightly lightened in color.

Gradually fold the dry ingredients into the wet ingredients using a silicone spatula. Mix only until no streaks of flour remain to avoid gluten over-development.

Fold in the chocolate chips until evenly dispersed throughout the dough.

Cover the bowl tightly with plastic wrap and refrigerate for a minimum of 2 hours. This allows the flour to hydrate and the fats to solidify, ensuring a chewy texture.

Preheat the oven to 350°F (175°C) and line two large baking sheets with parchment paper or silicone baking mats.

Scoop the chilled dough into balls (approximately 1.5 tablespoons each) and place them on the baking sheets spaced 2 inches apart.

Bake for 10 to 12 minutes. The cookies are finished when the edges are barely golden brown, while the centers still appear soft and slightly underbaked.

Allow the cookies to cool on the baking sheet for at least 10 minutes to set before transferring them to a wire rack for final cooling.