Soak cedar plank in water for at 1-2 hours.
In a small bowl combine soy sauce, brown sugar, vinegar, garlic, grated ginger, and red pepper flakes, whisk until sugar is dissolved. Pour 1/2 of the marinade over fish until well coated. Marinade in refrigerator for up to 2 hours.
Heat a small saucepan on medium low heat and add the remaining marinade mixture. Whisk in the cornstarch until dissolved. Simmer sauce for 3-5 minutes or just until thickened.
Heat charcoal or gas grill on medium heat, Place soaked cedar plank on top of flames. Heat plank for 1-2 minutes then flip and heat another 1-2 minutes. Place salmon skin side down on cedar plank. Season with salt and pepper. Grill over medium heat for about 12-15 minutes. Or until the salmon is just opaque in the center and flakes with a fork.
Brush some extra sauce on, add salt and pepper and a squeeze of lemon if desired. Serve over rice.