In a large pot, boil potatoes in salted water until tender. Remove from water. When cool, cut into bite-size pieces. Cook bacon til crisp, place on paper towel, and cut into bit-size pieces when cooled.
In a bowl, mix together yogurt, mayonnaise, yolk, mustard, and cayenne; salt and pepper to taste. In a large bowl, gently combine potatoes, celery, pickles, scallions, bacon, and dressing