Preheat the oven to 400°F. Bring a deep pot of salted water to a boil. This will be used to cook the pasta.
In a deep saucepan or dutch oven, begin making your béchamel/white sauce. Add in the butter and let melt over low heat. Once melted down, slowly start adding the flour while continuously whisking. Add a little bit at a time and whisk until fully combined, about two minutes.
Add the pasta into the boiling pot of water and cook for about two minutes less than the package instructions. Drain once ready and set aside.
Add about half of the warm milk to the béchamel sauce pan. Raise the flame to medium heat to bring the milk to a simmer. Whisk continuously for about three minutes to thicken. Add in the rest of the warm milk, salt, black pepper, and freshly grated nutmeg and continue to whisk for another four minutes to thicken the sauce.
Add in the cheese and mix until melted into the sauce over very low heat. Once the cheese has melted down, add in the pasta and mix until the macaroni is fully coated by the cheese sauce.
Pour the macaroni and cheese into a baking dish. Spread evenly over the baking dish and place into the oven. Cook for fifteen minutes.
In the meantime, if topping with pancetta, begin frying the pancetta or bacon until crispy. Set aside to use for topping.
After 15 minutes, lower the oven to 350 and continue to cook the macaroni and cheese for another ten minutes.
Remove the baking dish from the oven and sprinkle the fried pancetta or bacon on top. Place back into the oven for another five minutes.
Remove the baking dish from the oven and enjoy with a sprinkle of freshly chopped parsley on top. Salute and cin cin!