Use exact measurements and level off the tops of measuring cups/spoons for best results.
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. (Anything left over is your treat!) 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. Immediately after you pull the tray from the oven, use a metal spatula to press the edges of the cookies inward if needed.
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.
Notes
Note 1: If using unsalted butter, add a heaping 1/8 teaspoon of salt. Make sure to measure butter correctly—often the wrapping on butter isn’t attached evenly, so cut from the middle of the stick if possible to make sure you only have 3 tablespoons.Note 2: When separating the yolk from whites, take your time to make sure no amount of white makes it in the dough. Also, get rid of the stringy part (chalaza); I gently pull it off with my fingers.Note 3: Accurate flour measurement is crucial. Use a food scale to weigh out exactly 62 grams for best results. Too much flour leads to cakey cookies, while too little causes excessive spreading.Storage: Store cookies in an airtight bag at room temperature. If you have unbaked dough, keep it in an airtight bag in the freezer and bake directly from freezer.