Allow the butter and filet mignon steaks to reach room temperature, approximately 30 minutes.

Prepare the garlic herb butter by mixing together softened butter, chopped rosemary, chopped thyme, garlic powder, and salt in a bowl until well combined. Place the mixture onto parchment paper, shaping it into a log. Wrap it up and refrigerate.

Brush the filet mignon steaks with olive oil on both sides and season generously with salt and pepper.

Place the seasoned steaks in the air fryer basket. Cook at 400°F for 9-10 minutes for rare, 10-11 minutes for medium-rare, 11-12 minutes for medium, 11.5-13 minutes for medium-well, or 14 minutes for well-done. Use a meat thermometer to ensure desired doneness.

After cooking, let the filet mignons rest for 3-5 minutes. Serve each steak with a slice of the chilled garlic herb butter on top.