Linea baking dish with parchment paper and then spray it with cooking spray. Set aside.
In a microwave-safe bowl, add the chocolate and butter and microwave on high heat for about 2minutes, stirring after every 30 seconds.
Remove from microwave and set aside to cool.
Now, in a bowl, add the sugar, egg, vanilla extract, and salt and whisk until creamy and light.
Add the chocolate mixture and whisk until well combined.
Add the flour and walnuts and mix until well combined.
Place the mixture into the prepared baking dish and with the back of spatula, smooth the top surface.
Slide the Air Fryer Basket inside and adjust the temperature to 355 °F to preheat for 5 minutes.
After preheating, arrange the baking dish into the Air Fryer Basket.
Slide the basket into Air Fryer and set the time for 20 minutes.
After cooking time is finished, remove the baking dish from Air Fryer and set aside to cool completely.
Cut into equal-sized squares and serve.