In large saucepan, over low heat, combine butter and cream, stirring occasionally until butter is melted. Add mushrooms, cooked prawns and Parmesan, stir, then cover and simmer 15 minutes, or until sauce begins to thicken.
While sauce is simmering, bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain. Toss sauce with hot pasta and serve.