Bring a large pot (5-6 quarts) of water to a boil and add 2 tablespoons salt.
In a large bowl, whisk together the egg yolk, cream, 1/2 cup Parmesan and 1/2 teaspoon salt. Grate the zest from the lemon with a rasp grater (a Microplane is my top pick) over the bowl, then slice the lemon in half and squeeze in the juice from one half. Whisk to combine.
Cook the pasta until al dente, usually 8 - 10 minutes. Taste and test!
Drain the pasta and immediately add to the bowl with the lemon cream. Top with the butter, then use 2 forks or a pair of tongs to gently toss the pasta until it's coated with the sauce. Cover the bowl and let it sit 5 minutes to thicken.
Serve hot, sprinkled with parsley, additional grated Parmesan and lemon zest, if you like.