Place the potatoes in a large pot of cold water. Add the salt and bay leaf and thyme and bring to a boil. Lower heat to a simmer and cook until potatoes are tender, about 25 minutes. Drain and set aside to cool.
Combine mayonnaise, buttermilk, egg, dill, mustard, celery, scallions and shallot in a large bowl and mix well. Season to taste with salt and pepper.
When the potatoes are completely cool, gently fold them into mayonnaise mixture until evenly coated. Cover and refrigerate until ready to serve.