Soak the cedar plank in salted water for 2 hours, then drain. Remove the skin and any remaining bones from the salmon fillet. Rinse the salmon under cold running water and pat dry with paper towels. Generously season the salmon with salt and pepper on both sides. Lay the salmon on the cedar plank and carefully spread the mustard over the top and sides. Place the brown sugar in a bowl and crumble between fingers, then sprinkle over the mustard. Set grill for indirect grilling and heat to medium-high. Place the cedar plank in the center of the hot grate, away from the heat. Cover the grill and cook until cooked through, around 20 to 30 minutes. The internal temperature should read 135 degrees F. Transfer the salmon and plank to a platter and serve right off the plank.