Soak cedar plank in water for 1-2 hours, then drain. Remove skin from 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 what was skin-side down) on the cedar plank, salt and pepper, crumble the brown sugar over the fillets, then spread the two mustards over the top and sides. I like a seed and a Dijon mixed but either would do. Make sure to get the mustard all over the fillets. You want to coat them well so it can create a delicious carmelized crust.
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.
---If you do not have a grill, or its winter and you don't want to go outside you can bake these in the oven. 450 degrees for 12-15 minutes. Your house will smell like a spa once the cedar warms up! Divine!