1. Melt the butter and chocolate together in a heavy-bottomed pan on low flame while stirring; then, let it cool to room temperature.
2. Preheat your Air Fryer to 360 F (180 C).
3. In a mixing bowl, whisk together egg with the sugar, vanilla extract, and a pinch of salt until you get a mixture that is light and creamy. Whisk in the cooled chocolate mixture and then add in the flour.
4. Get a baking dish that can fit into the Air Fryer basket and line with parchment paper and then spread evenly the brownie mixture. Sprinkle with chopped hazelnuts.
5. Set the Air Fryer timer to 18-20 minutes and cook the brownie at 360 F (180 C) until the top is nicely crispy. The inside should still be soft.
6. Allow the brownie cool in the baking dish. Then, cut it into about 8 pieces.