Prep the steak. Pat the steak cubes dry with paper towels. Removing surface moisture helps them brown better.
Season generously. In a bowl, toss steak with olive oil, salt, pepper, smoked paprika, garlic powder, onion powder, and red pepper flakes if using. Let it sit for 10–15 minutes at room temperature.
Preheat the air fryer. Set it to 400°F (200°C) for 5 minutes. A hot basket kickstarts browning.
Make the garlic herb butter. Mash softened butter with garlic, parsley, thyme, lemon zest, salt, and pepper. Set aside at room temp so it stays spreadable.
Arrange in the basket. Lightly spray the air fryer basket with oil. Spread the steak bites in a single layer with a bit of space between pieces. Work in batches if needed.
Air fry. Cook at 400°F (200°C) for 4–6 minutes, shaking the basket halfway. For medium-rare, start checking at 4 minutes. For medium, go 6–7 minutes. Time can vary by air fryer and size of the cubes.
Toss with butter. Transfer hot steak bites to a bowl and immediately toss with the garlic herb butter so it melts and coats every piece.
Finish and serve. Add a squeeze of lemon if you like, sprinkle with flaky salt, and garnish with extra parsley. Serve right away.