Steam the potatoes over a pan of boiling, salted water, covered with a lid, for about 20 minutes until they are tender.
Meanwhile, make up the dressing. Begin by crushing the salt quite coarsely with a pestle and mortar, and then add the garlic. As you crush it and it comes into contact with the salt, it will quickly break down into a puree. Next, add the mustard powder and really work it in, giving it about 20 seconds of circular movements to get it well blended. After that, add some freshly milled black pepper.
Now add the vinegars and work these in in the same way. Then add the oil, switch to a small whisk and give everything a really good whisking. As soon as the potatoes are cooked, cool them in a mixing bowl for 10 minutes, then stir in the vinaigrette and shallots while the potatoes are still just warm. Now add the chives and give everything a good toss to distribute any dressing collected in the base of the bowl and transfer the salad to a serving bowl.
You will also need a steamer.