Salt the pasta water, add pasta, and cook for 1 minute less than the package directions. Scoop out 1 1/2 cups of pasta cooking water, then stir 1 cup of water into the puree. Turn off heat. Drain pasta. Toss cheeses, pasta, and 3/4 of the pancetta into pan with the pumpkin mixture. Season with salt and pepper. Add more pasta water if needed to loosen the sauce. Serve in shallow bowls and top with the remaining pancetta, parsley, and toasted pumpkin seeds or hazelnuts, if using.