Cook fettuccini according to directions.
Meanwhile, in a saucepan, melt butter over medium-low heat.
Stir in the cream, 1/4c. parmesan cheese, Romano cheese, egg yolk, salt, pepper, and nutmge.
Cook and stir over medium-low heat. (DON'T LET IT BOIL!)
Drain fettuccini, top with the sauce, and remaining parmesan.