1. Mix the flour, sugar, cocoa powder, baking powder, and salt in a bowl. Then, combine egg and butter until even, and pour into the dry ingredients. Stir and mix until the batter is even.
2. Grease a small round cake pan with non-stick spray, and turn the batter into it. Then, smoothen the top and cook in the air fryer for 18 minutes at 350F. Then, take the pan out, and leave it cool for 10 minutes before slicing it into square pieces.

You can also view this clip for more ideas on air fryers.