Boil the 4 potatoes - save the water (Potato Water)
Mash potatoes, mix together with both cheeses, salt and pepper, and mint. This will be your filling.
Separately, mix the flour, salt, yolks, and potato water. This will be your dough.
Turn said dough out onto floured surface and knead until smooth. Let Stand, covered, for 10min.
Divide into 10-15 parts and roll each out until it's 1/4in thick. Cut into rounds and continue rolling until 1/8in thick.
Place 1tsp of the filling (potato mixture) into each dough round, moisten the edges a bit, and pinch together to close into a dumpling shape. Boil them in water for 4-5 min, or until they bob to the surface.
Drain them and sprinkle with finely chopped bacon and onions or buttered bread.
Makes 20 good sized servings.