Combine the buttermilk marinade ingredients (except for the chicken) in a large resealable container.
Add the chicken and toss to coat. Cover and refrigerate for at least 12 hours.
Combine all of the seasoning mix ingredients. Divide into two equally-sized batches.
Fire up your charcoal grill with the Vortex in the center. Make sure the lid and bottom air vents are open.
Whisk together the coating ingredients and place into a large resealable bag.
Whisk together the egg wash ingredients and place into a pie pan or large bowl.
Working in batches, remove the chicken from the marinade and shake off the excess.
Sprinkle with one half of the seasoning mix.
Transfer to the egg wash and coat well.
Shake off the excess and add to the baggie. Seal and shake to coat.
Arrange chicken around the edges of your grill (once the coals are hot and ashed over). Cover and cook for 45 minutes, rotating the lid every 15 minutes.
The last 5 minutes of cooking lightly brush the tops of the chicken with the oil to help crisp the skin even more.
Once chicken is done (165 F as measured in several places), remove and let cool slightly before serving.