Lightly grease a 20cm round cake pan. Line the base with baking paper.
Use a balloon whisk to whisk together the flour and sugar in a bowl. Make a well in the centre. Add the egg and Nutella. Use a large metal spoon to stir until combined. Transfer to the prepared pan and smooth the top.
Preheat the airfryer to 160C. Bake the brownie for 40 minutes or until a skewer inserted in the centre comes out with a few crumbs sticking. Set aside to cool completely.
Dust with cocoa powder, if using, and cut into pieces to serve.
