Between 24 and 48 hours before cooking, rub the kosher salt all over the turkey, under and over the skin and in the cavities.
Place it in a roasting pan, cover tightly with plastic wrap and refrigerate until ready to cook.
The day you plan to serve the turkey, remove it from the refrigerator about 1 hour before roasting.
Using paper towels, dry the turkey inside and out.
Rub the softened butter all over the skin.
Set the turkey breast side down on a roasting rack inside of a roasting pan just big enough to hold the turkey.
Meanwhile, preheat the oven to 400 degrees F. Roast the turkey for 1 hour, then reduce the heat to 350 degrees F. Continue to roast, basting the turkey occasionally with the pan juices, for 1 more hour.
Remove the turkey from the oven and carefully turn the bird breast side up.
Continue to roast until the temperature reads 165 degrees F in the thickest part of the breast away from the bone or 170 degrees F in the thickest part of the thigh away from the bone.
The turkey juices should run clear.
If the breast skin is not brown enough, increase the temperature in the oven to 500 degrees F and roast for 15 minutes until browned.
Remove the turkey from the oven, transfer to a carving board, loosely tent with foil, and let rest for 15 minutes while you prepare the gravy from the pan juices.