Preheat oven to 350°F. Add butter to a ceramic/porcelain bowl and microwave until just melted but not hot. Let butter and bowl fully cool to room temperature.

To the cooled butter, add both brown and granulated sugars. Stir briskly with a fork until well combined. Add the egg yolk and vanilla. Continue to whisk with a fork until the mixture is emulsified and appears very creamy.

Add baking soda, salt (if using unsalted butter), and cornstarch. Mix thoroughly with a fork.

Add flour and chocolate chips. Continue stirring with a fork until it becomes difficult, then switch to a silicone spatula and fold until the dough is uniform without dry streaks.

With the dough, fill up a 1/4 cup measuring cup and level off the top. Roll into a large ball. Repeat until you have 3 dough balls. Place the balls on a parchment or silicone-lined baking sheet. Freeze for 5–10 minutes while the oven heats.

Bake for 10–12 minutes or until the edges are lightly browned and the tops are set but soft looking. Press the edges of the cookies inward if needed immediately after pulling from the oven.

Optionally, press additional chocolate chips into the tops and sprinkle with sea salt. Allow cookies to cool on the baking sheet for 5 minutes before enjoying.