Pat the turkey dry with paper towels. Rub 3 tablespoons of butter all over the bird. Rub the salt over the bird evenly. Place the turkey into a large plastic bag, preferably a sealable one, press out the air, and seal it tight. Place the turkey in a roasting pan, breast side up in the refrigerator for 2 to 3 days. Turn the turkey at least once a day, and rub the salt into the bird through the plastic bag.