Preheat your oven to 225°F (107°C). In a bowl, mix together the smoked chipotle powder, ground allspice, kosher salt, black pepper, and olive oil to form a paste. Rub the spice paste all over the beef rib roast, ensuring an even coating. Allow the roast to sit at room temperature for about 30 minutes to 1 hour.

Place the seasoned roast on a wire rack over a baking sheet. Insert a meat thermometer into the thickest part of the meat, avoiding the bone. Roast in the preheated oven until the internal temperature reaches 125°F (52°C) for medium-rare, about 3-4 hours depending on the size of the roast.

While the roast is cooking, heat a large skillet over medium heat. Add the sliced leeks and a pinch of salt. Cook, stirring occasionally, until the leeks are soft and caramelized, about 15-20 minutes. Stir in the minced garlic and cook for an additional 2 minutes. Remove from heat and let cool slightly.

In a bowl, combine the softened butter, caramelized leeks, fresh thyme leaves, lemon zest, and lemon juice. Mix until well combined. Season with salt and pepper to taste.

Once the beef rib roast reaches the desired temperature, remove it from the oven and let it rest for at least 20 minutes before slicing. To serve, slice the roast into thick pieces and place on a platter. Top with generous dollops of caramelized leek butter and garnish with chopped parsley.