In a medium-sized mixing microwaveable bowl, add the 4 tablespoons butter, 1/2 cup granulated sugar, and 1/4 cup unsweetened cocoa powder Microwave at 30-second intervals, stirring in between each time until the mixture is smooth.
Mix in the 1/2 teaspoon salt, 1/2 teaspoon pure vanilla extract and 1 large egg mix well.
Add in the 1/4 cup all-purpose flour, and mix well, so that all of the ingredients are blended in.
Spray your brownie pan with non-stick cooking spray or olive oil spray.
Pour your batter into the prepared pan, spread so it is level.
Set into the air fryer temperature to 320 degrees F, and set cook time for 12-16 minutes. Use air fryer setting.
Let cool, for about 10-15 minutes before cutting and serving.