Line a 7-inch round or square cake pan with a parchment sling, leaving overhang on two sides. Preheat the air fryer to 310°F.
Whisk the melted butter, granulated sugar, and brown sugar together for a full minute until glossy.
Whisk in the eggs, egg yolk, and vanilla until the mixture lightens slightly.
Sift in the cocoa powder, then add the flour and salt. Fold just until no dry streaks remain.
Fold in 1 cup of the chocolate chunks.
Scrape into the prepared pan and spread level. Scatter the remaining ½ cup of chunks over the top and press in lightly.
Air fry at 310°F for 28–34 minutes, until the edges are set and a toothpick inserted in the center comes out with moist crumbs. Tent with foil if the top browns early.
Cool in the pan for at least 30 minutes, then lift out by the parchment and slice into 9 squares.