In a large pot of boiling salted water, cook the pasta, stirring often, until al dente. Drain, reserving 2 cups of the pasta cooking water.
12 ounces dry fettuccini, Kosher salt
Set a large skillet over medium-low heat and add 1 cup of pasta cooking water. Bring just to a low simmer, then whisk in butter, one tablespoon at a time, until melted.
2 cups reserved pasta cooking water, ¾ cup unsalted butter
Gradually add the cheese, whisking constantly, being sure it is melted and incorporated thoroughly before adding each addition.
1 cup finely grated Parmesan cheese
Whisk in the additional 1 cup of pasta cooking water.
Add the pasta and mix to coat until the noodles are covered in sauce.
Taste and season with kosher salt and freshly ground black pepper, if desired.
Freshly ground black pepper
Cook over low heat, about 3-5 minutes, just until the pasta absorbs most of the liquid. Stir often to avoid scorching the cheese-covered pasta.
To serve, place ¼ of the pasta on a plate and add 1 sliced chicken breast. Top with a heaping helping of grated Parmesan cheese and garnish with chopped fresh parsley or basil leaves.
4 cooked boneless, skinless chicken breasts, Chopped fresh parsley