Remove the prime rib roast from the refrigerator and allow it to come to room temperature for about 1 hour.

Preheat your smoker to 225°F (107°C).

In a bowl, combine the smoked guajillo chili powder, crushed toasted cumin seeds, kosher salt, black pepper, minced garlic, olive oil, chopped rosemary, and thyme. Mix until a paste forms.

Rub the spice mixture all over the prime rib, ensuring it is evenly coated. Let it sit for 30 minutes to absorb the flavors.

Place the seasoned prime rib in the smoker and smoke for about 4 to 5 hours, or until the internal temperature reaches your desired doneness (130°F for medium-rare).

Remove the roast from the smoker and let it rest for at least 20 minutes before slicing.

In a mixing bowl, combine the softened unsalted butter, mashed black garlic, chopped parsley, lemon juice, salt, and pepper. Mix until well combined.

Transfer the compound butter to a sheet of plastic wrap, shape it into a log, and refrigerate until firm.

Slice the prime rib against the grain and serve with slices of black garlic compound butter on top.