In a small bowl, combine softened butter, minced garlic, parsley, lemon zest (if using), ¼ teaspoon salt, and ¼ teaspoon pepper. Mix thoroughly until evenly incorporated. Place on plastic wrap, form into a log, and refrigerate for 15 minutes to firm slightly.

Pat chicken breasts dry with paper towels. Check for even thickness; if portions are significantly thicker than others, place between plastic wrap and gently pound to even thickness of approximately 3/4 inch.

Brush chicken breasts lightly with olive oil on all sides. Season with remaining salt and pepper, ensuring even coverage.

Using a sharp knife, cut a horizontal pocket into the thicker side of each chicken breast, being careful not to cut all the way through. The pocket should extend to within 1/2 inch of the opposite edge.

Remove butter from refrigerator, slice into 1/4-inch disks. Insert 1-2 disks (depending on chicken size) into each chicken pocket. Secure with toothpicks if necessary.

Set air fryer to 375°F and allow to preheat for 3 minutes.

Place prepared chicken breasts in the air fryer basket, ensuring they're not touching. If cooking multiple pieces, maintain adequate space between them for proper air circulation.

Cook for 6 minutes, then flip chicken breasts. Continue cooking for 6-8 additional minutes, depending on thickness.

Check internal temperature using a meat thermometer inserted into the thickest part of the breast. The chicken is done when it reaches 165°F.

Remove chicken from air fryer and let rest for 5 minutes before serving to allow juices to redistribute.