Wash, pare, and cut potatoes in one-third inch cubes,there should be three cups; parboil three minutes, and drain. Add one-third cup butter, and cook on back of range until potatoes are soft and slightly browned. Melt two tablespoons butter, add a few drops onion juice, two tablespoons flour, and pour on gradually one cup hot milk. Season with salt and paprika, then add one egg yolk. Pour sauce over potatoes, and sprinkle with finely chopped parsley.