In a large kettle, cook potatoes in boiling salted water until tender. Drain and cool. Peel potatoes, cut into small chunks. Separate 5 egg yolks from whites. Set yolks aside. Chop whites and add to potatoes with onion.

In a small bowl, mash yolks. Stir in mayonnaise, milk, vinegar, mustard, sugar, salt and pepper. Pour over potatoes; toss well. Adjust seasonings if necessary. Spoon into a serving bowl. Garnish with egg slices from remaining egg and paprika. Chill until serving.