Trim and prepare roast (if needed): If roast came with a thick fat cap, trim fat cap so that it is about 1/4-inch thick. If prime rib roast has bones, you may choose to cut them away from the roast and secure them back into place with kitchen twine. Otherwise, bones can remain intact and be sliced away after the roast has finished cooking. If roast is wet, pat dry with paper towels.
Salt roast: At least 1-8 hours before smoking prime rib roast, cover the roast with kosher salt and place, uncovered, in the refrigerator.
Make compound butter: Place herbs and garlic in a food processor and blend well. Add softened butter and blend until mixture is uniform.
Prepare grill and smoke: Preheat smoker grill to 225-250°F. Coat top and sides of roast with compound butter. Insert meat thermometer into center of roast. Place roast on rack over baking sheet if saving drippings, otherwise plan on placing roast directly on grill.
Smoke prime rib: Place prepared prime rib roast on center of grill. Smoke until internal temperature reaches 110°F. This may take around 30 minutes per pound, but time can vary dramatically depending on starting temperature of roast and consistency of grill temperature.
Rest: Once the roast reaches 110°F, remove the roast from the grill but leave the thermometer in place. Rest the roast by wrapping in peach butcher paper; otherwise, loosely tent with foil. The temperature will rise 5-8°F during 30min rest period. As soon as the roast is removed from the grill, increase heat to the highest set point, about 400-450°F. At this time you may use drippings for other use.
Reverse sear: After a 30-minute rest, remove the roast from the foil or the butcher paper and place directly back on the hot grill. Cook until the roast reaches a temperature that is about 5°F below your ideal temperature at the center of the roast (recommended temperature to remove from the grill: 125°F). Keep in mind that the pieces on the end of the roast will be more done. Final temperature ranges: rare: 120-130°F, medium rare: 130-135°F, medium: 135-145°F, medium well 145-155°F, well done: 155°F. Allow roast to rest for an additional 10-15 minutes before slicing.
Final rest: Allow roast to rest for an additional 10-15 minutes before slicing. Serve with horseradish cream sauce and Au Jus.