Mix Dry Ingredients: In a medium bowl, whisk together flour, baking soda, cornstarch, and salt.
Combine Wet Ingredients: In a large bowl, whisk the melted butter, brown sugar, and granulated sugar until combined. Add the egg, egg yolk, and vanilla extract, and mix well.
Form Dough: Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Fold in the chocolate chips or chunks.
Chill Dough: Cover the dough and chill in the refrigerator for at least 2–3 hours, or up to 3 days.
Preheat Oven: Preheat the oven to 325°F (163°C). Line baking sheets with parchment paper.
Shape Cookies: Scoop about 3 tablespoons of dough for each cookie and roll into balls. Place them on the prepared baking sheets, spacing them appropriately.
Bake: Bake the cookies for 12–13 minutes, or until the edges are lightly browned. The centers will look soft and underbaked.
Cool: Allow the cookies to cool on the baking sheet for 10 minutes before transferring them to a wire rack to cool completely.