Place potatoes in pan and cover with water. Bring to a boil.
Reduce heat; cover and simmer for 20-25 minutes or until tender. Drain.
When cool enough to handle, peel and cube potatoes. Place in a large bowl.
Add the hard cooked eggs, celery, onion, salt and celery seed; set aside.
In a small saucepan, combine the sugar, cornstarch and mustard. Stir in
cream, egg and vinegar until smooth. Cook and stir over medium heat until
mixture coats the back of a metal spoon. Cook and stir 1-2 minutes longer
or until thickened.
Remove from the heat; let stand for 20 minutes. Whisk in mayonnaise and
butter until smooth. Cool slightly. Pour over potato mixture; mix
well. Cover and refrigerate for at least 6 hours.
10-12 servings