Ensure that the potatoes are washed, peeled and washed again. Bring to the boil covered with water and seasoned with salt. Simmer gently until potatoes are soft then drain off the water and cook over a low heat until dry. Pass through a drum sieve and stir in the yolk and butter and season with salt and pepper to taste. Pipe into 20mm ball with a 5mm ball on top on a lightly greased tray. Bake in a hot oven for 2-3 minutes then brush with egg wash. Brown lightly in a hot oven or under a salamander. Serve.