Optional marinade: Trim fat and remove tendons (try this easy hack!). Place chicken in a container or bag, pour buttermilk over, and seal. Marinate 20 to 30 minutes (or up to overnight). Discard buttermilk and pat chicken dry.
In a shallow bowl, stir together panko and Parmesan. In a second bowl, whisk remaining ingredients into a thick batter. Using tongs, coat each tender in batter, then press into the panko mixture, using most of the breading. Generously spray one side with cooking spray.
Air Fryer: Heat air fryer to 400°F. Place tenders in a single layer (cook in batches, no more than 5 at a time). Cook 5 to 8 minutes, flip, spray the other side, and cook another 5 to 8 minutes until golden and internal temp reaches 160°F. Carryover heat brings it to 165°F. Check smaller tenders early.
Oven: Preheat oven to 400°F. Generously spray a large sheet pan. Place tenders spaced apart on the pan. Bake 10 minutes, flip, spray the other side, and bake 7 to 10 more minutes until internal temp reaches 160°F.
Make dipping sauce if using. Serve hot. These are best fresh and lose crispiness as they sit.
Notes
Note 1: If using chicken breasts, remove excess fat, pound to even thickness, and slice into strips about the size of tenders (around 3/4-inch thick). One pound of tenderloins is about 6 to 8 pieces.Note 2:Panko creates a lighter, crunchier coating than regular breadcrumbs. Find it near breadcrumbs or flour.Note 3:Use freshly grated Parmesan, not shelf-stable. It melts better and has much better flavor and texture.Note 4: Dipping saucesChicken Dipping Sauce: Whisk until smooth: 1/2 cup mayonnaise 1/4 cup ketchup 1/4 teaspoon Worcestershire sauce 1/2 teaspoon black pepper 1/2 teaspoon garlic powder 1/2 teaspoon white sugar, optional 1/4 teaspoon Tony Chachere's Creole seasoningHoney Mustard: Whisk until smooth: 1/4 cup + 2 tablespoons mayo 1-1/2 teaspoons yellow mustard 3/4 teaspoon lemon juice 1-1/2 tablespoons honey 1 tablespoon honey BBQ sauce 1/4 teaspoon fine sea salt 1/8 teaspoon pepperNutrition Note: Calories do not include dipping sauces.Storage: These are best fresh. For make-ahead, wait to bread in the panko-Parmesan mixture until just before baking for the crispiest result.