Bring a large pot of lightly salted water to a boil. Cook pasta for 8 to 10 minutes, or until al dente. Drain, and set aside.

In a small saucepan, bring 2/3 cup cream to a simmer, stirring often. Whisk in bouillon and cheese until well blended and bouillon has dissolved completely. Dissolve cornstarch in water, and whisk into mixture. Cook and stir till the cream thickens, then remove from heat and set aside.

Melt butter in a medium skillet over medium high heat. Stir in prosciutto, garlic, and mushrooms; cook until mushrooms are tender, about 3 minutes. Add tomato, remaining cream, and parsley flakes; heat through.

To serve, place pasta in a large mixing or serving bowl. Add the mushroom mixture and pour in cream sauce. Toss well, and serve.