Whereas pulled pork, a similarly indulgent roast, uses pork butt, pernil capitalizes on the thick skin layer and subcutaneous fat of the bone-in shoulder. In some places, this might be hard to come by and will have to be ordered from the butcher. Score the skin with a sharp knife to a depth of half an inch in a crosshatching pattern, and peel away the fat and skin layer but don’t separate it completely. Rub seasoning all over the skin, working it into the cuts, as well as beneath the loose skin layer, which can be laid back in its original position. Place the shoulder in a glass dish, cover with shrink-wrap and marinate overnight in the fridge. On roasting day, allow the shoulder to come back up to room temperature, and place skin-side up on a rack inside a high-sided roasting tray.