Bring a large pot of water to boil. Add in a drizzle of olive oil & pinch of sea salt to water. Cook pasta for 8-10 minutes. Reserve ¼ cup pasta water and drain the rest.
Mix in the milk & season with salt and pepper. Add in the 4 tbsp of butter (in cut up chunks), stir around until its melted throughout. Sprinkle the flour all over the pasta and mix. Add in the shredded cheddar and combine that into the pasta. Lastly, rip apart your slices of American cheese into smaller pieces and mix them in.
Everything should be wet and cheesy! 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.