Mix in the milk & season with salt and pepper. Add in the butter (in cut up chunks), stir around. Rip apart your slices of American cheese into smaller pieces and mix them in. Add in the shredded cheddar and combine that into the pasta.
A sauce will have been created right into the pasta, if it seems a little dry, add in some of the reserved pasta water 1 tbsp at a time. You can top with anything from seasoned breadcrumbs, extra shredded cheese, bacon, scallion, etc!
This will keep and reheat, but stir in some milk after its hot to revive the sauce.