Peel the potatoes and add to a large pot of boiling salted water. Cook until tender but still firm, about 15 minutes. Drain, cool and cube. Place into a large bowl and set aside.
In a double boiler, bring vinegar to a boil. Gradually beat in eggs and cook over simmering water until thickened. Stir in butter and cook 1 minute. Whisk in cream, parsley, onion, mustard, salt, pepper and cayenne. Pour over hot potatoes and combine. Cool, sprinkle with parsley and refrigerate until serving time.