While water comes to boil, bring 1 cup heavy cream and butter to simmer in 3- to 4-quart saucepan over medium heat; reduce heat to low and simmer gently until mixture reduces to 2/3 cup, 12 to 15 minutes. Off heat, stir in remaining 1/2 cup cream, 1/2 teaspoon salt, and pepper.

Add 1 tablespoon salt and pasta to boiling water; cook pasta until just shy of al dente. Reserve 1/4 cup pasta cooking water, then drain pasta. Meanwhile, return cream mixture to simmer over medium-high heat; reduce heat to low and add pasta, Parmesan, and nutmeg to cream mixture. Cook over low heat, tossing pasta with tongs to combine, until sauce coats pasta and pasta is just al dente and cheese is melted, 1 to 2 minutes. Stir in reserved pasta cooking water; sauce may look rather thin but will gradually thicken as pasta is served and eaten. Toss pasta to coat well with sauce. Serve immediately.