Bring a large pot of salted water to a boil. Cook the fettuccine until al dente, about 8 to 10 minutes. Reserve 1 cup pasta water, then drain.

In a large skillet over medium heat, melt the butter. Add the garlic and cook, stirring, until fragrant, about 30 seconds.

Pour in the heavy cream and simmer, stirring often, until slightly thickened, about 2 minutes.

Remove from heat, stir in the Parmesan a handful at a time until smooth. Add 1/4 cup pasta water and toss with the drained pasta. If needed, add more pasta water to loosen. Season with salt and pepper, garnish with parsley, and serve right away.