Flip the chicken breast-side down. Using kitchen shears, cut along both sides of the spine and remove it. Flip the chicken breast-side up and firmly press down to crack the breastbone so the bird lays flat.
Rub chicken all over with the salt and sugar mixture. Let IT sit uncovered for 25 minutes at room temperature.
Liberally brush both sides of the chicken with half of the prepared marinade. Refrigerate uncovered overnight. Reserve/refrigerate unused marinade.
The next day, preheat the oven to 285°F (140°C). Place chicken on a rack-lined tray and roast for 1 hour and 15 minutes. The internal temperature should reach 135–140°F (57–60°C) in the breast.
Increase oven temperature to 475°F (245°C). Brush the bird liberally with reserved marinade (about ¼ cup). Roast for 15 minutes, then brush again lightly and roast an additional 10–15 minutes until deeply browned. Rotate bird during the final 5 minutes for even caramelization.
While the chicken finishes roasting, pour the remaining marinade into a saucepan. Bring to a boil and simmer for 10 minutes to reduce and sterilize. Stir frequently.
Once chicken reaches 165°F (74°C) in the breast and 175°F (80°C) in the thigh, remove from oven. Brush one final time with the cooked sauce and let rest for 10–15 minutes. Carve and serve.