Bring a well salted pot of water to a boil. Once it reaches a boil, add then bucatini and cook according to the package. Reserve 1/4 cup of the pasta water then drain the pasta, and toss back into the pot. Stir in the cold butter. Once the butter has been fully incorporated, sprinkle in the black pepper and lemon zest, toss together, then stir in the lime juice and arugula. Serve with parmesan cheese and chili flakes.