Remove the corned beef brisket from its packaging. Rinse it thoroughly under cold water and pat it dry with paper towels. This removes excess surface salt so the finished brisket is well-seasoned but not overly salty.
Tear off a large sheet of heavy-duty aluminum foil, big enough to fully wrap the brisket. Place the corned beef in the center of the foil. Sprinkle the contents of the enclosed spice packet all over the brisket, then pour the beef broth around the base of the meat.Wrap the foil tightly around the brisket, sealing the edges completely so no steam escapes. This creates a braising environment inside the air fryer.
Place the foil-wrapped brisket in the air fryer basket. Cook at 300 degrees F for 60 minutes.
Carefully flip the foil packet over and cook for another 50 to 60 minutes at 300 degrees F. The brisket should be fork-tender and the internal temperature should be around 200 degrees F for that pull-apart texture.
In a small bowl, mix together the dijon mustard, brown sugar, garlic powder, onion powder, and black pepper to create a glaze.
Carefully open the foil and transfer the brisket to a plate. Drain the cooking liquid but save it for serving if desired. Place the brisket back in the air fryer basket unwrapped. Spread the mustard-brown sugar glaze evenly over the top and dot with butter.
Cook at 390 degrees F for 5 to 6 minutes until the glaze is bubbly and caramelized into a golden crust on top.
Remove the corned beef and let it rest for 10 minutes. Slice against the grain into thin slices and serve.