24 hours before roasting (or up to 48 hours), use kitchen shears to remove the wingtips. Cut through the first wing joint (see notes). Remove the giblet bag from the chicken cavity, then season the entire chicken generously with salt, inside and out. Let it rest for 30 minutes at room temperature.
Stir 2 tablespoons of kosher salt into the buttermilk until it's dissolved. Place the chicken in a gallon-size resealable plastic bag, then add the salted buttermilk.
Seal the bag and turn over several times to coat the chicken, then place on a rimmed plate or a casserole dish (to prevent leaking). Place in the refrigerator and chill for 24-48 hours. Turn the bag over periodically to help the chicken marinate evenly (once or twice is plenty).
Remove the chicken from the fridge an hour before roasting so it can come to room temperature. Place an oven rack on the center shelf, then preheat the oven to 425 degrees F.
Remove the chicken from the bag and discard the leftover marinade. Use paper towels to gently blot off the excess buttermilk without disturbing the skin. Stuff the chicken cavity with the onion, garlic and herbs (if using), then use butcher's twine to truss the chicken legs (this helps the chicken cook evenly). Season the outside liberally with ground black pepper.
Place the chicken on a roasting pan, legs up. (Note: I recommend using a roasting pan with an insert that prevents the chicken from sitting directly on the bottom).
Place the roasting pan in the oven on an angle, pointing the legs toward the rear left corner with the breasts near the center of the oven (this will help prevent the breasts from overcooking since the back of the oven is more hot).
After 15 minutes, when the chicken is beginning to brown, reduce the heat to 375 degrees F. Adjust the pan so the legs face the opposite back corner of the oven. Cook for an additional 45-60 minutes, until the skin is completely brown and a digital thermometer registers 165 degrees F in the thickest part of the thigh. The juices should run clear. If the legs ever begin looking like they're darkening too much while roasting, you can rotate the pan so the breasts are in the back of the oven.
Once the chicken is fully cooked, remove it from the oven and let it rest for 15 minutes before carving and serving.
Leftovers cam be stored in an airtight container in the refrigerator for 3-4 days.
If you don't trim the wing tips, they'll char while roasting and potentially create a burning smell.Fresh herbs are optional and will add some additional flavor to the meat. You can stuff as many herbs into the chicken cavity as will fit along with the onion and garlic. I added 6 sprigs of thyme.Adapted from Salt Fat Acid Heat by Samin Nosrat