Cut sirloin into 1-inch cubes and trim excess fat.
In a bowl, combine olive oil, kosher salt, chili powder, black pepper, 1/4 tsp garlic powder, onion powder, and brown sugar to form a rub.
Toss beef cubes in the spice mixture until evenly coated.
Preheat the air fryer to 400°F for 3 minutes.
Arrange the beef in a single layer in the air fryer basket; do not overcrowd.
Air fry for 8 minutes, shaking the basket at the 4-minute mark.
Check that the internal temperature reaches 135°F for medium-rare (use a meat thermometer).
While the beef cooks, melt butter and stir in 1/4 tsp garlic powder, parsley flakes, and red pepper flakes.
Remove beef from the basket and let rest for 3 minutes.
Toss rested beef with the garlic butter and serve immediately.