Put the butter, pancetta and mushrooms in a large skillet and saute until golden. Add the tomatoes and toss everything together. Add the cream and simmer over medium heat until the cream is thick and coats the back of a spoon, about 5 to 8 minutes. Season with salt and pepper.
Cook the pasta in a large pot of boiling salted water. (The ratio is 1 teaspoon of salt to 1 quart of water.) Drain the pasta but do not shake all the water out; it should be moist, not wet. (See note.).
Toss the pasta, pine nuts and Parmesan in a large, warm bowl. Add the sauce and mix again. Serve in warmed pasta plates or basin-shaped plates. Sprinkle on the pecorino cheese and add a bit more pepper to garnish.