Prep the pan and air fryer: Line your pan with parchment and lightly grease. Preheat the air fryer to 320°F (160°C) for 3–5 minutes.
Melt butter and chocolate: In a microwave-safe bowl, heat butter and semi-sweet chocolate in 20–30 second bursts, stirring until smooth. Set aside to cool slightly.
Whisk sugars and eggs: In a medium bowl, whisk granulated sugar, brown sugar, and eggs for about 1 minute until slightly thick and glossy. Whisk in vanilla.
Combine wet ingredients: Slowly stream the melted chocolate mixture into the egg mixture while whisking until fully combined.
Fold in dry ingredients: Sift in flour, cocoa powder, and salt. Gently fold until just combined. Do not overmix. Fold in chocolate chips if using.
Fill the pan: Spread the batter into the prepared pan, smoothing the top. The batter will be thick.
Air fry: Place the pan in the air fryer basket. Cook at 320°F (160°C) for 16–22 minutes, depending on pan size and thickness. Start checking at 15 minutes. The edges should look set, and a toothpick inserted 1 inch from the edge comes out with moist crumbs. The center should look slightly soft and glossy.
Cool for texture: Let brownies cool in the pan for 20–30 minutes to finish setting. For clean slices, cool to room temperature and chill for 30 minutes.
Slice and serve: Lift out with parchment and cut into squares or wedges. Enjoy warm for maximum gooeyness.