If you are going to make a meat loaf, preheat the oven to 350°F. Mix the mashed potatoes, beef, egg yolks, milk, beets, onion, capers, salt, and pepper. Shape to fit a loaf pan, and bake 45 minutes. Or, shape the mixture into six patties. Melt the butter and oil in a skillet and fry the patties over medium heat, turning as they brown. Or, preheat the broiler. Melt the butter and oil in a skillet and press the mixture flat with a spatula. When it has formed a crust on the bottom, place under the broiler to brown the top. Serve in pie-shaped wedges.