Butterfly salmon; remove head, tail and fins. Run knife down backbone of fish until it opens flat, careful not to cut through.
Coat foil lightly with cooking spray. Place salmon skin-side down on foil. Spread softened butter over flesh of salmon. Drizzle with lemon juice and dry mustard. Cover with brown sugar, using more sugar if needed.
Place foil with fish on barbecue grill over low heat, cover. Bake for 20 to 30 minutes. Salmon is cooked when flesh flakes easily.