Preheat air fryer to 390 degrees for 4 minutes.
Trim any fat from chicken breast, Slice into 1/2 inch thick slices, then each slice into 2 to 3 nuggets. Season chicken pieces with salt and pepper.
Place melted butter in a small bowl and breadcrumbs ( with Parmesan, if using ) in another small bowl.
Dip each piece of chicken in butter, then breadcrumbs.
Place in a single layer in the air fryer basket. Depending on the size of your air fryer, you may need to bake in two batches or more.
Set timer to 8 minutes.
When done, check if the internal temperature of chicken nuggets is at least 165 degrees F. Remove nuggets from basket with tongs and set onto a plate to cool.