Step 1Cut the pancetta into one-fourth-inch dice.
Step 2Heat the oil in a deep skillet over medium-low heat. Add the pancetta and cook, stirring, until it crisps and renders its fat, about 10 minutes. Remove all but about 1 tablespoon of the fat from the pan and discard. Add the onion and cook, stirring until tender and translucent, about 10 minutes. Add the red pepper flakes, then crush the tomatoes into the pan and add the puree. Cook, stirring to crush the tomatoes into the finest bits, for 5 minutes. Simmer 40 minutes, stirring occasionally.
Step 3While the sauce cooks, bring a large pot of water to a rolling boil with salt (about 1 tablespoon for every 2 quarts -- it should taste salty). Fifteen minutes before the sauce is done, add the pasta to the water. Stir and then cook until it is just al dente, 11 to 12 minutes or according to the package directions.
Step 4Reserving one-half cup of the cooking water, drain the pasta in a colander. Add the butter and toss quickly, then transfer to the pan with the sauce. Stir until the strands are thinly but evenly coated. Add a little cooking liquid if the pasta seems dry. Add half the ricotta salata and stir to combine well. Transfer the pasta to a large bowl or platter and sprinkle with the remaining cheese. Serve hot.