Immerse cedar planks in water as directed by manufacturer. Place salmon on plank. Sprinkle salmon with rub. Grill over indirect heat until salmon is cooked through and internal temperature is at least 150°F, about 20 to 25 minutes.
Drizzle asparagus with butter and sprinkle with rub. Grill until tender, about 3 to 5 minutes. Serve with salmon.