Mix well all the ingredients and let the dough rest in refrigerator for 1 hour. With the help of a rolling pin or a pasta sheet, roll out the pasta into very thin sheets. Place the sheet on a table and cut strips about 15 inches long & 1 1/2 inch wide.
For the filling:
In a small pot melt butter with flour, then add milk and bring to boil on low flame and keep stirring. Add Fontina and allow to cook for 5 min. Remove from flame, pour it into a bowl and let cool down. When cold make little balls of 1/2 inch diameter.
Set the Fontina filling on the pasta sheet making sure you have 1/2 inch space in between. Roll the sheet over to cover the filling and pinch every single ravioli to enclose the cheese in the pasta. With a pizza cutter, cut each ravioli and place them on a plate sprinkled with some flour.
For serving:
Bring a large pot of salted water to a boil. Drop in the ravioli and let them cook for 1 minute.
In the meantime, melt the butter in a sauce pan (and add the porcini if substituting); when ravioli are ready place them in the melted butter, season it with the Parmigiano and shave the truffle on top.