Prepare Dry Ingredients:
In a large bowl, whisk together the flour, baking soda, cornstarch, and salt. Set aside.
Combine Wet Ingredients:
In a medium bowl, whisk the melted butter with the brown sugar and granulated sugar until no lumps remain. Add the egg and egg yolk, whisking until smooth. Stir in the vanilla extract. The mixture will be thin.
Mix Dough:
Pour the wet ingredients into the dry ingredients and mix with a large spoon or rubber spatula until just combined. The dough will be soft and appear greasy. Fold in the chocolate chips. The chocolate chips may not stick well due to the melted butter, but try to incorporate them as best as you can.
Chill Dough:
Cover the dough tightly and chill in the refrigerator for at least 2–3 hours, preferably overnight. This step is crucial to prevent the cookies from spreading too much during baking.
Preheat Oven:
Preheat your oven to 325°F (163°C). Line large baking sheets with parchment paper or silicone baking mats.
Shape Cookies:
Let the dough soften slightly at room temperature for about 10 minutes. Use a cookie scoop or tablespoon to measure out 3 scant tablespoons (about 2 ounces or 60g) for XL cookies, or 2 heaping tablespoons (about 1.75 ounces or 50g) for medium/large cookies. Roll the dough into balls, shaping them so they are taller rather than wide, like a cylinder. This helps them bake up thicker. Place 8–9 balls of dough onto each baking sheet.
Bake:
Bake the cookies for 12–13 minutes, or until the edges are lightly browned. XL cookies may take closer to 14 minutes. The centers will look soft but will set as they cool. Allow the cookies to cool on the baking sheet for 10 minutes before transferring them to a wire rack to cool completely.
Cool and Serve:
While the cookies are still warm, press a few extra chocolate chips into the tops if desired for a more appealing look. Once fully cooled, store the cookies in an airtight container at room temperature for up to 1 week.
