With a sharp knife, puncture and score the skin of the ham, making parallel 1/2-inch-deep incisions. Turn the ham and score again to make a grid-like pattern.
In a small bowl combine the salt, Essence, pepper, and cayenne. Rub evenly all over the ham. Place the ham, fatty side up, in a large roasting pan. Place 1 clove where the lines intersect on the ham. Rub the sugar evenly over the ham, pressing to pack. Pour 2 cups of cola into the bottom of the roasting pan and bake for 1 hour. In a container, combine the whisky and the remaining 2 cups of cola.
Baste the ham with the whiskey and cola and continue to bake, basting every 15 minutes with the whisky and coke mixture and pan juices, until the ham is cooked through, the skin is crisp and dark, and a meat thermometer registers 165 degrees F, 2 to 2 1/2 hours.
Remove from the oven and let rest at least 20 minutes before carving. To serve, slice the meat thinly across the grain and arrange on a platter.