Whisk butter and flour in another medium-sized pot over medium heat until the paste smells like buttery, nutty toast.
Slowly add milk while whisking.
The paste will seize up at the very beginning,
but will be very liquid once all the milk is poured.
Cook until the mixture has the consistency of heavy/whipping cream, whisking constantly.
Add the shredded cheddar and mozarella cheese and stir until they are all melted.
Add salt, pepper, nutmeg and cayenne pepper to suit your taste.
Fold in the cooked macaroni and mix to incorporate with the cheese sauce.
Transfer into medium baking dish or into individual-sized baking pans. Top with parmesan cheese and panko bread crumbs, if using.
This recipe can be easily doubled to serve 4.