Combine dry ingredients. Whisk the flour, baking soda, cornstarch, and salt together in a large bowl. Set aside. ½ cup (60 g) all-purpose flour ¼ teaspoon baking soda ½ teaspoon cornstarch ¼ teaspoon salt
Combine wet ingredients. In a medium bowl, whisk the melted butter, brown sugar, and granulated sugar together until no brown sugar lumps remain. Whisk in the egg yolk and vanilla extract. 3 tablespoons (43 g) unsalted butter 3 tablespoons packed light brown sugar 2 Tablespoons granulated sugar 1 large egg yolk ¾ teaspoon vanilla extract
Combine wet and dry ingredients. Pour the wet ingredients into the dry ingredients and mix together with a large spoon or rubber spatula. The dough will be quite soft but thick.
Add chocolate. Fold in the chocolate chips (this part can be tricky but try to get them incorporated as best you can). ⅓ cups (58 g) semi-sweet chocolate chips or chocolate chunks
Scoop and bake. Roll the dough into balls, about 2 Tablespoons of dough each. The dough may be crumbly at first, but will come together as you roll it in your hands. Chill and refrigerate covered for at least 1 hour.
Prep oven and cookie sheets. Preheat oven to 325°F (163°C). Line a baking sheets with parchment paper or silicone baking mats. Set aside.
Bake the cookies. Bake the cookies for 10-12 minutes until the edges are just set but the middles are still soft. Allow to cool on the cookie sheet for 5 minutes before transferring to a wire rack to cool completely. Allow to cool on the cookie sheet for 5 minutes before transferring to a wire rack to cool completely.