Combine roast and chopped onion in a 4-quart slow cooker.
Combine cola and next 7 ingredients; reserve 1/2 cup in refrigerator. Pour remaining mixture over roast and onion.
Cook, covered, on HIGH 6 hours or until roast is very tender; drain and shred roast. Keep warm.
Combine reserved 1/2 cup cola mixture, ketchup, and butter in a small saucepan; cook mixture over medium heat, stirring constantly, just until thoroughly heated. Pour over shredded roast, stirring gently. Spoon onto buns, and serve with potato chips and pickle spears, if desired.