Soak a large untreated cedar plank (about 3/4-in. thick) in water for 30 minutes. Wipe off water and rub the plank with 1 teaspoon vegetable oil. Season a 2 to 2 1/2 lb. salmon fillet with 1/4 teaspoon salt and 1/8 teaspoon pepper. Place fillet, skin side down, on the plank and dot with 1 teaspoon butter. Place on a medium-hot grill (you can hold your hand at grill level 4 to 5 seconds) and cover with grill lid. Cook salmon until center of fillet flakes, about 25 to 30 minutes. Check flame level frequently to make sure the plank does not ignite.