Bring a large pot of salted water to a boil. Prepare the pasta according to the package instructions, cooking until al dente. Reserve 1 cup of the starchy pasta water, then drain the pasta.
Melt the butter in a large skillet over low heat. While whisking, pour in the cream. Add the garlic and salt and cook, whisking occasionally, for 5 to 8 minutes, or until thickened. Add half the Parmesan and whisk until melted.
Add the pasta and toss to coat, then add the remaining cheese and toss again until it melts into the sauce. If the sauce becomes too thick, add the reserved pasta water as needed to loosen it. I typically add around ½ cup.
Season to taste and serve topped with freshly ground black pepper and parsley.
Notes
We recently refreshed this post with an updated recipe and photos. For the cauliflower version, see this cauliflower Alfredo sauce recipe.