Boil peeled potatoes. Mash them. Remain them cool. In a large bowl mix well potatoes, flour, egg yolk and salt. On board is dusted flour and put the dough on top. Shape it roll. Cut is at a slight inclination of the pieces neither too large nor too small. If you put sticky flour. On cloth dusted flour and put the pieces of dough. In a large saucepan put to boil water. Gnocchi are placed in the water. When you come to the surface and are ready to uproot them with a slotted spoon. Put them in a large bowl. In a pot or pan put the butter, tomatoes and water. It is left until a sauce. Add grated parmesan and can even butter. Gnocchi are served with sauce.