Preheat oven to 400 degrees F.
Skewer the baking potatoes with metal skewers, so they cook through faster and place them directly on the middle oven baking rack for 1 hour and 15 minutes.
Meantime add the sour cream to a bowl with the whole-grain mustard, salt, white pepper and chopped chives, leaving a teaspoon back to sprinkle on top. Stir all the ingredients together and scatter over the remaining chives. If you are not eating it straight away store it covered in plastic wrap in the refrigerator.
When the potatoes are ready with a crisp skin and soft inside, remove them from the oven, take them off the skewers and cut a cross in the top of each of them and slightly squeeze them so they open out and place them into bowl. Serve the potatoes with the chive sour cream on the side and have people help themselves.