Bring a large pot of cold water to a boil over high heat and salt it generously. Add the pasta and boil, stirring occasionally, until al dente, about 12 minutes. Drain pasta in a colander, leaving 3 to 4 tablespoons pasta water in the bottom of the pot.
Return pot to a low heat. Whisk the butter in bit by bit, letting each piece melt completely before adding the next, to make a creamy sauce. Toss the pasta in the sauce. Season with salt and pepper and nutmeg if desired. (Add a little warm water if the pasta looks shiny.) Transfer to warmed bowls and serve warm, with Parmesan sprinkled over each dish if desired.
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