Whisk the flour, cornstarch, baking soda, and salt in a big bowl. Set the bowl aside.

In another bowl, melt your butter, then whisk it up with the granulated and brown sugar until you've got a smooth mixture. Crack in one egg (plus egg yolk), whisk it well, and add vanilla extract.

Pour that thin, wet mix (from Step 2) into your dry ingredients (Step 1) and stir everything with a rubber spatula or big spoon.

Toss in those chocolate chips.

Cover your dough and refrigerate it for 2 to 3 hours. For even better results, chill it overnight (or 2-3 days)!

Take that dough out and let it soften up a bit at room temperature (about 10 mins). Preheat your oven to 325°F (or 163°C) and line your baking sheets.

Grab a tablespoon or small cookie scoop and portion out about 3 tablespoons of dough (for extra-large cookies) or about 2 tablespoons (for medium to large ones).

Roll them up into balls, but make sure they're taller than they are wide. Space them out on your baking sheets, about 8 to 9 balls a sheet.

Bake your cookies (12 to 13 minutes). If you're making extra-large cookies, they might need a minute or so longer. When they're done, the edges should just start to turn a light golden brown, but the center parts will still look super soft.

(Optional) While the cookies are still warm, press some chocolate chips onto the tops to make them look extra pretty.

Let your cookies sit on your baking sheets for about 10 minutes. Then, transfer them over to a rack to chill completely. Enjoy!