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Aside from the obvious -- the bone-in chicken -- the apple cider vinegar is perhaps the most important part of the Cornell chicken marinade. It adds a unique "tang" and a depth of flavor that defines Baker's BBQ recipe. Adding the egg also allows the skin to crisp over, adding a great texture to bite into.
In a deep dive into online discussions on Cornell chicken, I found multiple testimonies in a thread on the r/Wings subreddit that Chiavetta's Barbecue Marinade is based on the Cornell chicken recipe, and is a fantastic store-bought alternative marinade to provide similar results.
Source: cnet.com