In a medium, heavy-bottomed saucepan, gently warm the heavy cream over low heat.
When tiny bubbles form at the edges, begin whisking in the Parmesan cheese, a small handful at a time, until it's completely melted and the mixture is smooth.
Continue whisking constantly over low heat for about 10 minutes.
Add the butter, one piece at a time, allowing each to fully melt before adding the next.
Once all the butter is incorporated, the sauce should be velvety and smooth.
Remove the pan from the heat.
In a separate small bowl, lightly beat the egg yolks. Slowly drizzle about a half-cup of the hot cream sauce into the yolks while whisking constantly to temper them.
Pour the tempered egg yolk mixture back into the saucepan, whisking vigorously to combine.
Season to taste with salt and white pepper.
As the sauce finishes, bring a large pot of heavily salted water to a rolling boil.
Add the fettuccine and cook until al dente, following the package .
Drain the pasta quickly in a colander (do not rinse) and transfer it to a large, warmed serving bowl. Immediately pour the hot cream sauce over the top.
Using tongs or two large spoons, gently lift and toss the fettuccine until every strand is evenly coated in the luscious sauce.
Garnish with chopped fresh parsley and serve right away.
Keep the heat on low throughout the sauce-making process. High heat can cause the cream to separate or the cheese to become grainy.
Reserve a cup of the starchy pasta water before draining. It's perfect for thinning a sauce that has become too thick, as the starch helps the sauce cling to the noodles.
