How to Make It
Soak 2 (11- x 5- x 1/4-inch) cedar grilling planks in water 1 hour. Drain; set aside.
Stir together chili sauce, vinegar, brown sugar, mustard, Worcestershire sauce, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper in a small saucepan. Bring to a boil over medium-high; boil, stirring constantly, 1 minute. Remove from heat. Pour half of the mixture into a small heat-proof bowl, and reserve for serving. Set aside remaining mixture in saucepan.
Preheat a gas grill to medium (350°F to 400°F). Place grilling planks on unoiled grates; let stand until planks are charred on bottoms, about 3 minutes. Flip planks. Place 3 salmon fillets, skin side down, on each plank. Sprinkle salmon evenly with remaining 1 teaspoon salt and 1/2 teaspoon pepper; brush with chili sauce mixture in saucepan. (Discard remaining mixture in saucepan.) Grill, covered, until desired degree of doneness, about 20 minutes for medium-rare. Serve with reserved chili sauce mixture in bowl.