In a large mixing bowl, combine the room temperature butter, dark brown sugar, and granulated sugar. Beat on medium speed until smooth and creamy, approximately 2-3 minutes. Add the eggs and vanilla extract, mixing until well incorporated, and occasionally scrape down the sides of the bowl.

Sprinkle the cornstarch, baking soda, salt, and oats over the wet mixture. Mix until just combined. Gradually add the flour, ensuring it is spooned and leveled before measuring. Mix until just incorporated, being careful not to over-mix. Gently fold in 1 cup of the milk chocolate chips with a spatula.

The dough will be quite sticky at this point. Cover the bowl and refrigerate for about an hour. If chilling for more than a few hours, allow the dough to sit at room temperature for 15-20 minutes before rolling into balls.

Preheat your oven to 350°F. Prepare baking sheets with parchment paper or silicone baking mats.

Shape the dough into balls (approximately 1.7 ounces each if using a food scale), making them taller rather than wide. Place the dough balls on the prepared baking sheets. Bake for 8-11 minutes, watching closely towards the end to avoid over-baking. The centers might appear slightly underdone, but they will firm up as they cool.

Allow the cookies to rest on the baking sheet for a few minutes before transferring them to a wire rack to cool completely. While the cookies are still warm, press the remaining chocolate chips into the tops of the cookies for an extra chocolatey bite and a visually appealing finish. Optionally, sprinkle a tiny amount of Fleur de Sel on top if you enjoy a sweet and salty combination.