Spray inside of 6-quart slow cooker with no stick cooking spray. Add potatoes, stock and bay leaves. Cover.
Cook 4 hours on HIGH or until potatoes are tender, stirring after each hour. Remove bay leaves. Stir in butter, salt, pepper and garlic powder. Cover. Let stand 5 minutes.
Beat potatoes with electric mixer on medium-high speed or mash with potato masher, gradually adding milk, until smooth. Stir in parsley. Top with additional butter, if desired.