Old-Fashioned Potato Salad with Cooked Dressing (No Mustard!)

Ingredients

• 1/2 cup milk
• 1 teaspoon table salt
• 1/4 cup granulated sugar
• 1 tablespoon cornstarch
• 1/4 cup vinegar
• 1 egg
• 2 tablespoons butter (softened)
• 1/4 cup onion (chopped)
• 3/4 teaspoon celery seed
• 1/4 cup mayonnaise
• 1/4 cup sweet pickle relish
• 7 medium potatoes, diced and cooked
• 3 hard-cooked eggs, diced