Bring duck breasts to room temperature for 20-30 minutes before cooking to ensure even cooking.
Score the duck skin in a diamond pattern about 0.25 inch deep without cutting into the meat. Space cuts about 0.5 inches apart to allow fat to render.
Dry brine the duck breasts by seasoning both sides evenly with kosher salt. Place skin side up on a plate and rest uncovered in the fridge for at least 30 minutes. Before cooking, pat the skin dry completely with paper towels.
Preheat the air fryer to 400°F. Lightly brush olive oil over the duck skin and season with black pepper and five-spice powder. Place the breasts skin side down in the air fryer basket with at least 2 cm space around each piece.
Air fry the duck skin side down for 12 to 15 minutes. Flip the breasts skin side up and cook an additional 3 to 5 minutes until the skin is golden and crispy. Use a thermometer to check for an internal temperature of 135°F for medium-rare.
Remove the duck breasts from the air fryer and rest under loose foil for 5 to 10 minutes to redistribute juices. Slice against the grain at a slight angle before serving.