Mix the dry ingredients together in one bowl. Then mix the wet ingredients in a large measuring cup. Add the wet to the dry and mix well.
Add in the mix-in(s) of your choice and mix again.
Preheat your air fryer to 350°F. Line a 5-inch round cake or pie pan (or a loaf pan that fits in your air fryer) with parchment paper. (Alternatively you can spray the pan with oil, but then the brownies will not be oil-free.)
Place the pan in the fryer basket or on the tray if you have an oven-style air fryer. Cook for 20 minutes. If the middle is not well set or a knife doesn't come out clean when stuck in the middle, cook for 5 minutes more and repeat as needed. Cooking time may vary depending on the size of your pan and your particular air fryer.
Let the brownies cool for about 30 minutes before cutting.