Separate a large egg and place the egg yolk in a large bowl (discard or save the white for another use). Add the very soft unsalted butter, granulated sugar, packed light brown sugar, and vanilla extract to the bowl.

Using a sturdy spoon, mix the yolk, butter, sugars, and vanilla vigorously for about 90 seconds, or until the mixture is smooth and slightly aerated.

Add the old-fashioned whole-rolled oats (do not use instant or quick-cook), all-purpose flour, baking soda, cornstarch, and kosher salt to the bowl. Stir until the dry ingredients are fully incorporated and a cohesive dough forms.

Stir in the 1/3 cup semi-sweet chocolate chips until evenly distributed through the dough.

Shape the dough into a single ball with your hands. Flatten the ball so the mound is about 1 1/2 inches thick (about halfway flattened).

Wrap the dough mound tightly in plastic wrap and chill in the refrigerator for at least 2 hours to reduce spreading and keep the cookie thicker. (If you skip chilling, the cookie will spread more.)

About 10–15 minutes before baking (after chilling), preheat the oven to 350°F. Line a baking sheet with parchment paper or a silicone baking mat, or lightly grease the sheet.

Place the chilled dough mound on the prepared baking sheet. If desired, press a few additional chocolate chips onto the top of the dough mound before baking.

Bake for 12 to 15 minutes, checking at 12 minutes. The cookie is done when the edges are set and the top is just set — it may still look slightly pale and glossy in the center. Bake closer to 12 minutes for a softer center, or up to 15 minutes for a more well-done cookie.

Remove the baking sheet from the oven. If desired, press a few more chocolate chips onto the hot cookie so they melt on top. Allow the cookie to cool on the baking sheet until set; the cookie will firm up as it cools.

Serve warm and fresh for best texture.