Cook the macaroni in plenty of boiling salted water, keeping them slightly firm. Rinse in fresh water and drain.
Beat the egg. Finely chop the white mushrooms and sweat in a small frying pan, without letting them brown, until all the liquid has evaporated. Add the chopped shallot and mascarpone and cookfor 2 to 3 minutes longer over medium heat.
Transfer the mixture to a bowl, add the bread crumbs, beaten egg, one-third of the grated parmesan and the finely crushed truffle. Season with salt and pepper and a few gratings of nutmeg. Mix well to form a thick paste.
With a piping bag, fill the macaroni with the truffle mixture.
Divide the macaroni into six portions and place on a buttered baking sheet. Sprinkle with the remaining parmesan and place in a very hot oven or under the broiler until browned.