The day before cooking, season the chicken generously with 2 teaspoons kosher salt and let it sit for 30 minutes. Optional: tightly tie the legs together with a piece of butcher’s twine.
Place the chicken in a large mixing bowl large enough to hold it and pour in the buttermilk. Cover tightly and refrigerate overnight, rotating the chicken halfway so both sides get marinated.
Remove the chicken from the refrigerator 1 hour before you plan to cook it, then remove it from the buttermilk, shake off excess and discard the buttermilk.
When ready to cook, transfer the chicken to the air fryer basket (at least 6 quart) belly side down and season the top of the chicken with 1/2 teaspoon salt.
Air fry at 350F for about 25 minutes, until the top of the chicken is browned; flip the chicken over, season the other side with the remaining 1/2 teaspoon salt and cook another 25 minutes, until golden, crisp and brown all over and the juices run clear when you insert a knife down to the bone between the leg and the thigh.
Remove the chicken to a platter and let it rest for 10 minutes before carving.
To bake in the oven, bake at 400F until golden, crisp and brown all over and the juices run clear when you insert a knife down to the bone between the leg and the thigh, about 1 hour or longer if needed.