Cook the pasta in plenty of salty boiling water ‘al dente’. Heat the cream in a saucepan and simmer for one minute. Drain the cooked pasta, add the butter, and toss. Add the pasta to the cream, stirring several minutes over low heat. Season and add the Mémoire truffle cheese, tossing again to coat the pasta evenly. Serve directly from the pan with freshly ground black pepper and watercress.