Lightly grease a small, air fryer-safe baking pan, mug, or ramekin with a bit of butter or non-stick cooking spray.
In a microwave-safe bowl or a small saucepan and melt the 2 tablespoons unsalted butter. Once melted, add 3 tablespoons granulated sugar 2 tablespoons unsweetened cocoa powder 1/4 teaspoon vanilla extract and a 1/8 teaspoon kosher salt. Mix well.
Once the mixture is well combined and slightly cooled, stir in the 1 large egg yolk Make sure the mixture isn't too hot to avoid cooking the yolk.
Add the4 tablespoons all-purpose flour to the mixture and stir until combined. Be careful not to overmix, as it could make the brownie dry.
If you use 2 tablespoon chocolate Chips or chopped chocolate, fold them in now.
Pour the brownie batter into your prepared pan or ramekin.
Set the air fryer to temperature to 350°F (175°C) and set cook time for10-12 minutes. Using air fryer setting.
Allow the brownie to cool for a few minutes before enjoying it right from the pan. Be careful as it will be hot!