Butterfly the chicken by placing breast side down and using heavy duty kitchen shears to cut along both sides of backbone. Discard backbone, turn chicken over and open it like a book. Press gently to flatten.
Place chicken in a large freezer bag. Add buttermilk, ¼ cup oil, garlic, peppercorns, salt, rosemary, and molasses. Seal bag securely and refrigerate overnight, or up to two days.
Preheat the oven to 400°F. Remove chicken from marinade and place on a rack with a paper towel underneath, so excess liquid can drip off.
Line a roasting pan with foil and place the rack with the chicken in the pan. Drizzle with remaining 2 tablespoons of oil.
Roast for 20 minutes, then reduce heat to 325°F. Continue roasting until well browned and juices run clear when chicken is pierced where leg joins thigh, approximately 15-20 minutes.
Place chicken on a carving board and allow to rest for 10 minutes before cutting into serving pieces.
Drizzle with pan juices over the chicken before serving.