The day before you are going to cook the chicken, generously season the whole bird with salt and let sit for 30 minutes.
Stir 2 Tablespoons of salt into the buttermilk. Place the chicken in a freezer bag or plastic shopping bag (double up if using these) and pour the buttermilk in. Move the chicken around a bit to coat. Zip up/tie the bag and place on a rimmed baking sheet. Place in refrigerator for 24 hours.
Remove the chicken from the refrigerator about 45 minutes before roasting so it can come up to room temperature. Preheat oven to 425 degrees.
Remove chicken from bag and wipe off excess buttermilk. Place chicken in a cast iron skillet or roasting pan. Then place the skillet/pan on the center rack of the oven towards the back. Rotate the pan so the legs are pointed towards the back left corner of the oven.
Roast for 20 minutes, turn the oven temperature down to 400 degrees and rotate the chicken so that the legs are pointed to the back right corner of the oven.
Roast for about 40 minutes longer. The chicken should be browned all over and the juices run clear when a knife is inserted to the bone between the leg and thigh.
Remove chicken from oven and let cool at least 10 minutes before carving.