Marinate Chicken: Whisk the Spice Mix ingredients together in a small sealable bag or container. Add 1 TB Spice Mix, 1 cup buttermilk, and chicken to a large freezer bag. Marinate in the refrigerator for 6–24 hours. Store remaining Spice Mix separately.
If baking (RECOMMENDED): Preheat the oven to 400°F. Line a half-rimmed baking sheet (18×13-inch) with parchment paper (foil will NOT work because the chicken sticks to it). Add the butter and melt it in the oven while it preheats, checking every few minutes. Remove the baking sheet once the butter has melted.
Meanwhile Mix Breading: Mix the flour, panko, cornmeal, and remaining Spice Mix in a large bowl. Add half of this breading mixture to a freezer bag.
Bread Chicken: Remove the chicken from the buttermilk and dab off the excess marinade with paper towels (I plop the chicken on paper towels). Add half the chicken to the breadcrumb bag and shake until well coated. Press the breadcrumbs into the chicken as you remove the tenderloins from the bag.
If baking, line the breaded chicken on the buttered baking sheet. Repeat with the remaining chicken.If air frying, line the breaded chicken on parchment paper. Repeat with the remaining chicken. TO BAKE (Recommended): Bake at 400°F for 10 minutes. Flip the chicken over and bake for 5 minutes, until the internal temperature reaches 160°F. Broil to desired crispiness.
TO AIR FRY: Preheat the air fryer to 400°F. Working in batches, add the chicken to the air fryer basket in a single layer and lightly spray with cooking spray. Cook for 5 minutes on each side or until crispy and the chicken's internal temperature reaches 160°F.
Rest: Let the chicken rest for 5 minutes to rise to an internal temperature of 165°F. Serve with your favorite dipping sauces.