Preheat your air fryer to 320°F (160°C) and lightly grease an air fryer-safe baking dish to prevent sticking.

In a bowl, whisk together the melted butter and granulated sugar until smooth and combined.

Mix in the vanilla extract, and then add the eggs one at a time, ensuring each is mixed in well before adding the next.

In a separate bowl, sift together the flour, cocoa powder, salt, and baking powder.

Gradually add this dry mixture to the wet ingredients, stirring until just combined.

Pour the batter into the prepared dish, smoothing the top with a spatula for even baking.

Bake in the air fryer for 20-25 minutes. Check for doneness by inserting a toothpick; it should come out with a few moist crumbs attached.

Once done, allow the brownies to cool completely in the dish to set their structure nicely.

For the icing, whisk together the cocoa powder, melted butter, powdered sugar, milk, and vanilla until smooth and glossy.

Spread the icing evenly over the cooled brownies, top with rainbow sprinkles, and let it set before slicing into squares.