In a medium bowl, combine the melted butter, brown sugar, and granulated sugar. Whisk vigorously until the mixture is glossy and well-combined, about 30 seconds.

Stir in the egg yolk and vanilla extract until fully incorporated. The batter should look creamy and rich.

In a separate smaller bowl, whisk together the flour, baking soda, salt, and cornstarch if using.

Gradually add the dry mixture to the wet mixture, mixing with a wooden spoon or spatula until just combined. Fold in the majority of the chocolate chips, reserving about 1 tablespoon for pressing onto the tops later.

Using a small cookie scoop or two teaspoons, form 4 to 6 dough balls. They should be just shy of 2 tablespoons each. Press the reserved chocolate chips directly onto the tops of the dough balls.

Cut small squares of parchment paper slightly larger than the cookie dough balls. Place the parchment squares inside the air fryer basket, spacing the cookie dough evenly (usually 2 to 3 cookies fit comfortably, depending on size).

Cook at 310°F (155°C) for 8 to 10 minutes. Begin checking around the 8-minute mark. Your goal is a deeply golden edge with a slightly pale, still-soft center.

Immediately after removing the basket, sprinkle a tiny pinch of flaked sea salt over the warm cookies. Let them cool in the air fryer basket for at least 5 minutes to firm up.