Preheat the oven to 400 degrees.Drizzle the chile with a little olive oil.Roast for 15-20 minutes. The chile should be blistered all over. Remove from the oven and allow to cool.When the poblano is cool, remove the stem and seeds and any of the loose skin that has separated from the meat. Purée with the buttermilk, sour cream, lime, salt and cilantro until smooth. Keep refrigerated.