Preheat a large skillet over medium heat. Add the pancetta and cook, stirring occasionally, until crispy, 5 to 7 minutes. Set aside on a paper towel-lined plate.
Bring a large pot of salted water to a boil. Prepare an ice water bath. Stir the arugula into the boiling water and cook until it is bright green and tender, about 15 seconds. Remove the arugula directly to the ice bath and let cool completely, then drain, gently squeeze out the excess water and set aside. Do the same for the basil. Once the arugula and basil are blanched, cook the pasta in the boiling water until al dente.
Meanwhile, make the pesto. Add the arugula and basil to a food processor and blend until mostly pureed, about 30 seconds. Add oil, pine nuts, garlic and salt and pepper and puree for another 30 seconds. Scrape down the sides of the bowl, then add the cheese and pulse, drizzling more oil as desired, until combined but not fully pureed, another 30 seconds. (See Cook's Note.)
Place the pesto and butter in a large bowl and add drained pasta to the bowl, reserving some pasta water to thin the sauce as desired. Mix the pesto and pasta, season to taste and crumble the slices of crispy pancetta on top. Serve with extra grated Pecorino and a drizzle of olive oil.
Any extra pesto can be covered with layer of olive oil and sealed in an airtight container, then refrigerated up to 3 days or frozen up to 1 month.