Cover potatoes with cold water in a large pot. Bring to a boil and simmer until just tender, about 20 minutes. Drain and cool until you are able to handle them.While the potatoes are cooling make the dressing: place cheese and lemon juice into a food processor and process. Pour the buttermilk in a slow stream through the feed tube until it reaches the consistency you like. Taste and add salt as needed (and pepper if you haven't used the pepper variety of Boursin)Cut any larger potatoes in half and place all potatoes in a large mixing bowl with the celery and the capers. Pour the dressing over the potatoes, and using clean hands, gently mix everything together until it is well-coated.Transfer to a serving bowl, garnish with parsley and serve warm or at room temperature.