Step 1: Bring a large pot of salted water to a rolling boil.
Step 2: Add 1 pound of fettuccine pasta and cook according to package directions until al dente.
Step 3: Drain the pasta, reserving about 1/2 cup of pasta water, and set aside.
Step 4: In a medium saucepan or skillet, melt ½ cup (1 stick) of unsalted butter over medium heat.
Step 5: Add 3 cloves of minced garlic to the melted butter and sauté for 1-2 minutes until fragrant, being careful not to brown the garlic.
Step 6: Pour in 2 cups of heavy cream and bring the mixture to a gentle simmer. Do not boil.
Step 7: If using, add 2 ounces of cream cheese, stirring constantly with a whisk until it's completely melted and smoothly incorporated into the cream.
Step 8: Reduce the heat to low. Gradually add 1 cup of freshly grated Parmesan cheese and ½ cup of freshly grated Pecorino Romano cheese (or an additional ½ cup of Parmesan if not using Romano), whisking continuously until the cheese is melted and the sauce is smooth and creamy. If the sauce is too thick, add a tablespoon or two of the reserved pasta water until desired consistency is reached.
Step 9: Taste the sauce and adjust with salt and freshly ground black pepper as needed.
Step 10: Add the cooked fettuccine pasta directly into the sauce and toss gently to coat every strand evenly.
Step 11: Serve immediately, garnished with extra grated Parmesan cheese and a sprinkle of fresh chopped parsley, if desired.