Bring a large pot of water to the boil and salt generously. Add the noodles and cook until they are tender but not mushy.
Meanwhile, ladle 1/4 cup of the noodle cooking water into a medium skillet. Set the skillet over low heat and, while whisking constantly, gradually add the butter, piece by piece, (let each piece of butter melt into the sauce before adding the next bit) until a smooth sauce has formed. Stir in the parsley and salt and pepper to taste. Add the lemon zest if desired.
Drain the noodles in a colander set in the sink and leave whatever water clings to them, do not rinse. Transfer the noodles to a large bowl, add the sauce, and toss well with the sauce. Serve.
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