Cut the potatoes into even pieces or slices, and rinse under cold water. Boil potatoes in a pot of water until fork-tender, and drain. Return potatoes to the pot, and dry over low heat for a couple of minutes.  Fold in the rosemary, scallion, garlic, olive oil, and butter; season with salt and pepper; and mash the potatoes, then cool to room temperature. In a small non-stick skillet, add the bacon fat.   Add the mashed potatoes almost to the lip of the pan, and press down gently with your hand to form a pancake of sorts. Cook over medium-high heat until a crust forms on the bottom, about 5 to 10 minutes. Flip the potatoes in a pan or invert onto a plate and slide them back into the pan to cook the other side until a crust forms. Plate the potatoes, and sprinkle with truffle oil and parsley.