Place the potatoes and the kosher salt in an 8-quart pot and fill it with enough water to cover the potatoes. Bring the water to a gentle boil and cook the potatoes until tender. Drain. Peel the potatoes while they are still warm and slice them 1/2-inch thick.
Whisk together the mayonnaise, evaporated milk, vinegar, mustard, and sugar in a small bowl. In a large bowl, combine 1/3 of the potatoes, eggs, onion, and celery with about 1/3 of the mayonnaise mixture. Repeat with the remaining ingredients, combining them 1/3 at a time until they are well mixed. Season with salt and pepper. Refrigerate for 1 hour, then garnish with paprika and chives.
Whisk together the egg yolk, mustard, and lemon juice in a large stainless-steel bowl. Add a few drops of the oil and continue whisking. Gradually add the oil in a thin stream while continuing to whisk. Stop every few seconds and check that all the oil has been incorporated. Season with the cayenne and salt and pepper. Store in the refrigerator for up to 2 weeks.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.
Recipe courtesy David Page and Barbara Shinn, from Recipes from Home, Artisan 2001.