Preheat oven to 350 degrees. In a cast iron skillet heat the oil over high heat. Cook the potatoes whole, turning frequently until evenly browned, about 10 to 15 minutes. Transfer to paper towels to drain. When cool enough to handle, wrap each potato in aluminum foil and place in the oven. Bake until cooked through, about 40 minutes. Remove the foil and transfer the potatoes to a serving platter. Place a kitchen towel over the potatoes and crush with the palm of your hands. Sprinkle with salt and pepper and garnish with the crema, creme fraiche or sour cream. Serve immediately.
Whisk the cream and buttermilk together. Cover and set in a warm place (a gas oven with just the heat from the pilot light is fine) for 8 hours. Crema may be kept in the refrigerator as long as a week.