Soak a 1/2-in.-thick cedar plank in warm water at least 1 hour. Preheat oven to 450°. Rub salmon with olive oil and BBQ rub, set on plank skin side down, and roast directly on oven rack 8 to 12 minutes, or until almost opaque.
Melt butter in a medium frying pan over medium-low heat. Add crab and heat until warm.
Top each salmon piece with some crab and drizzle with beurre blanc.