Preheat air fryer to 350°F. Line a 6-inch square or round cake pan with parchment paper.
In a microwave-safe bowl, melt the butter and chocolate chips. Heat in 20-second increments, stirring until smooth.
Combine the melted chocolate mixture with the sugar and vanilla in a large bowl. Mix well. Whisk in the egg.
In a small bowl, mix together the flour, cocoa powder, and salt. Stir into the chocolate mixture until just combined.
Pour the batter into the prepared pan and smooth out to the edges. Place the pan into the air fryer basket.
Bake for 15-20 minutes. A toothpick inserted in the center should still have moist crumbs stuck to it. If the top of the brownies starts to get too brown while they are cooking, tent a piece of foil overtop.
Remove from air fryer and let sit in the pan for at least 30 minutes to set. Once cooled, cut into squares.
Notes
These brownies will keep in an airtight container in the refrigerator for up to 4 days, and 4 weeks in the freezer.