Boil the potatoes and rub through a sieve or run through an electric blender while hot. Stir in the butter and cheese. Mix in the flour, egg and the egg yolk. Season to taste with salt pepper.
Roll into small balls and poach in salted boiling water for 10 minutes. Drain and serve in almost any soup.
These make very acceptable family fare topped with warmed-up gravy leftovers.