For the Erdäpfelsalat (Prepare first):Place cubed potatoes in a pot, cover with water and salt. Place reserved skins in a fine-mesh strainer submerged at the top of the pot. Boil until tender (approx. 15 mins). Discard skins and drain potatoes. Transfer to a baking sheet, sprinkle with 2 tbsp white wine vinegar, and cool. In a large bowl, combine potatoes with oil, mustard, onion, chives, stock, and sugar. Stir gently to release starch and thicken the dressing. Rest for 30 mins (up to overnight) before serving.