2Put the mashed potato, flour, and baking soda into a bowl with plenty of salt and black pepper. Stir in the parsley, then add the yogurt or buttermilk, egg, and enough milk to mix to a soft but not sticky dough.
3Knead the dough briefly on a well floured surface, then lightly roll out to a thickness of 3/4 inch. Stamp out circles with a 2 inch plain cutter. Reroll the trimmings and continue until all the dough has been shaped.
4Heat the butter and oil in a large skillet. Add the potato cakes, cook in batches if needed, and Fry over medium heat until undersides are Golden. Turn over and cook the other side in the same way. If the skillet is ovenproof put it straight into the preheated oven. Alternatively, transfer the cakes to a baking sheet. Cook for 5 to 10 minutes, until the cakes are cooked through.
Meanwhile, line the broiler rack with foil and arrange the bacon and tomatoes on top. Tear the leaves from the Rosemary, chopped, and sprinkle over the tomatoes with a little salt and black pepper. Broil until the bacon is almost cooked, then drizzle with maple syrup and continue to cook until Golden.
5To serve, halve the potato cakes, spread with a little butter, and fill with pieces of bacon and halved tomatoes. Arrange on a platter and serve at once. Makes 4 to 6 servings.