Prepare the Dough

• Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
• In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt.
• In a large bowl, beat the softened butter, brown sugar, and granulated sugar until creamy and fluffy.
• Add the egg, egg yolk, and vanilla extract. Mix until smooth.
• Gradually add the dry ingredients, mixing just until combined.
• Fold in the chocolate chips until evenly distributed.

Shape the Cookies

• Scoop the dough into balls, about 2 tablespoons each.
• Place them on the prepared baking sheets, leaving about 2 inches between each cookie.
• For thicker cookies, chill the dough for 20–30 minutes before baking.

Bake the Cookies

• Bake for 9–12 minutes, until the edges are lightly golden but the centers still look slightly soft.
• Let the cookies cool on the baking sheet for 5–10 minutes before transferring them to a wire rack.
• Allow them to cool slightly before serving so the centers can set while staying soft and chewy.