Preheat the air fryer to 375°F.
Cut the top sirloin steak into 1-inch cubes, trimming off any excess fat if desired. Add the steak cubes to a medium bowl. Drizzle with vegetable oil and season with kosher salt and black pepper. Toss to coat evenly.
Arrange the steak cubes in a single layer in the basket of your air fryer. Depending on the size of your air fryer, you may need to cook in batches. Air fry for 6–8 minutes, flipping halfway through, or until the steak reaches your desired doneness.
While the steak is cooking, prepare the garlic butter. In a small saucepan over medium heat, melt the butter. Add the minced garlic and cook for 30 seconds to 1 minute, stirring constantly. Stir in the parsley and black pepper.
Transfer the cooked steak bites to the saucepan with the garlic butter. Toss to coat the steak evenly.
Garnish with extra chopped parsley and serve immediately.