Preheat oven to 425 F. Spread roast with butter. Place on rack in roasting pan. Roast until meat thermometer registers 135 F., approximately 40 to 50 minutes. Remove roast to serving platter. Tent with foil to keep warm.
In a small, heavy saucepan, prepare Bearnaise sauce as directed on package. Remove sauce from heat. Carve roast. Serve with Bearnaise sauce.