In a saucepan melt 4 tablespoons butter & whisk in the flour.
When flour is completely incorporated, gradually pour in the milk, whisking constantly. Keep whisking until milk comes to a boil, then simmer for 2-3 minutes & remove it from the heat.
Stir in nutmeg, salt & pepper to taste, & add 1/3 of the cheese.
Let this sauce cool until just warm, then stir in the beaten egg yolks & set aside.
Bring a large pot of water to a boil, then add salt & pasta & cook for 2 MINUTES LESS than the package instructions indicate, so that the pasta is very al dente, as it will continue cooking in the oven.
Preheat oven to 375 degrees F & butter the inside of a 13"x9" baking dish.
When pasta is done, drain it & toss with the remaining 2 tablespoons of butter, then spread half of the pasta in the bottom of the baking dish. Pour half of the sauce over it & sprinkle half of the remaining cheese over that. Repeat layering with the remaining pasta, then the rest of the sauce & finally the cheese.
Place the baking dish on the oven's middle rack & bake 20-30 minutes or until golden.
Great when served warm or at room temperature with a fresh green salad.