To Make the Crumb Topping (optional)

1. If making the optional crumb topping, place Ritz in a food processor to crush to a crumb, or smash in a baggie with the bottom of a pan. 
2. Add the shredded cheese and melted butter and pulse to combine or mix by hand.
3. Pour into a sauté pan and over medium heat, toast to a light brown and remove. 

To Prepare the Macaroni & Cheese

1. In a bowl, beat egg yolks with sour cream until smooth. Whisk in one cup of the whole milk and two tablespoons of melted butter. Set this mixture aside. 
2. Bring a large pot of water to a boil and salt the water with a teaspoon of salt. Keep the water hot.
3. In a medium saucepan, combine the remaining three cups of milk with the heavy cream and heat to hot, but not scorched. Once hot, leave on low to stay warm.
4. Bring the salted water back to a boil and add the dry pasta. Cook less than the package directions to a slightly hard and chewy state. Drain into a colander. The pasta will cook further in the cheese sauce.
5. In a large pot, melt remaining four tablespoons of butter over medium low and once melted, stir in the flour and cook and stir for three minutes.
6. Whisk in the hot milk and cream one third at a time, whisking vigorously after each addition.
7. Add in salt, white pepper, dry mustard, cayenne pepper and nutmeg. Whisk to combine.
8. Stir in eight ounces of the shredded cheddar, keeping four ounces for later.
9. Stir in the American cheese slices, peeled brie, Boursin and Fontina, whisking each cheese in before adding the next.
10. Ladle some of the hot cheese sauce into the bowl with the egg yolks, whisking quickly as you add each ladleful. (This tempers the egg yolks so they don’t cook and turn into scrambled eggs.)
11. Pour this mixture back into the cheese sauce, again whisking quickly.
12. Reserve two cups of this cheese sauce for reheating leftovers and adding to portions for a creamier texture.
13. Pour the drained pasta into the pan of cheese sauce and stir. Bring the burner to medium heat and cook and stir for about ten minutes to finish cooking the pasta and to absorb the cheese sauce.
14. Once the pasta is cooked, add the remaining four ounces of shredded cheddar.
15. Serve in bowls with (optional) crumb topping sprinkled on top.
16. Add some of the reserved cheese sauce as needed to add more creaminess to portions or to use to reheat leftovers.