Peel and cube potatoes into uniform chunks. Cover potatoes with water in a large pot. Cover pot with lid and boil until tender. Drain and cool
Whisk together eggs, sugar, cornstarch and salt in a saucepan. Whisk in milk, vinegar, and mustard. Cook over medium heat for 7 to 10 minutes or until thickened (be sure to continuously whisk so mixture doesn't burn). Remove from heat and stir in butter.
Refrigerate mixture to chill. Once chilled add mayo.
Toss together potatoes, celery, onion and celery seed. Gently fold in dressing.
Refrigerate until ready to serve.
