In the bowl of an electric mixer fitted with the flat beater, beat the butter on medium speed until smooth, about 1 minute. Add the grilling rub and lemon zest and beat until well blended, about 1 minute more. Spoon the butter mixture onto a sheet of waxed paper and form into a log about 1 inch thick. Twist the ends to seal the butter, then wrap in plastic wrap. Refrigerate for at least 1 hour or up to 2 days.
Soak a cedar plank in water for at least 20 minutes or up to 4 hours.
Prepare a medium fire in a grill. Have ready a spray bottle of water to extinguish any flare-ups on the plank. Bring the seasoned butter to room temperature.
Place the plank on the grill, close the lid, and heat until the plank begins to smoke and crackle. Rub the skinless side of the salmon with the potlatch seasoning. Place the salmon, skin side down, on the plank and arrange the lemon slices on top of the sh. Close the lid and grill until the salmon is cooked through, 8 to 10 minutes.
Transfer the plank with the salmon to a heatproof platter, or carefully transfer the salmon directly to a warmed platter. Cut as much of the seasoned butter as desired into slices 1/4 inch thick and place on the salmon, reserving any remaining butter for another use. Serve immediately. Serves 6 to 8.