Soak the dried porcini in plenty of warm water for 30 minutes.
Working on a board or in a bowl, make a mountain of flour, use your fist to swirl a crater and into it break the eggs, butter and vin santo. Use a fork to start and then your hands, to make a rough dough. Knead until firm and smooth, then cover with an upturned bowl and leave to rest.
Roll out by hand or with a machine and cut into 20cm squares. Bring a pan of salted water to the boil and blanch the sheets in batches for 1 minute, plunge them into cold water, then spread them out on clean cloths.
Drain the porcini, keeping the liquid. Slice the mushrooms.
For the béchamel, melt 50g of butter, add the flour and cook until golden and toasty. Add the warm milk and 400ml of porcini soaking liquid, a little at a time, beating well with a balloon whisk and then simmering until as thick as double cream.
In a frying pan, melt 20g of butter and the olive oil and fry the mushrooms until soft and collapsed, then add the soaked porcini. Add to the béchamel along with the cream and parsley, season, stir for a minute longer and then pull from the heat.
To assemble the vincisgrassi, butter a 2 litre ovenproof gratin dish and cover the bottom with a layer of pasta. Lay on some of the prosciutto, then spread over a layer of the béchamel, dot with butter and sprinkle with some Parmesan. Continue the process, making layer after layer, finishing with the seventh layer of pasta with defiant extruding corners, and top with the last of the béchamel and a final sprinkling of Parmesan.
Bake in the oven at 220°C for 20 minutes, then put under the grill for 3 minutes to toast the top.
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