Mac and cheese recipes typically get around this breakdown by stabilizing the cheese with a béchamel: the classic French white sauce made by cooking flour in melted butter to form a roux, then thinning out that starch-fat paste with milk until the mixture turns silky smooth. When you stir aged cheese into the sauce, starches from the flour wrap around proteins in the cheese, preventing them from squeezing out fat and recombining into slumpy curds. Some cooks even fortify the sauce with egg yolk, which adds emulsifiers that also help stabilize the emulsion.
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But my colleague, deputy editor Andrea Geary, found a much simpler way to make a sauce that’s both flavorful and silky-smooth in her Simple Stovetop Macaroni and Cheese: Replace the béchamel with American cheese. So-called process cheeses like American contain emulsifying salts, which not only keep the American cheese itself smooth when melted, but also change the nature of the proteins in the aged cheeses so that they can more effectively stabilize the fat-moisture network.
Even better, the whole ensemble comes together in one pot. All you do is simmer the macaroni in a combination of water and milk, which will form the base for the sauce; stir in shredded American cheese (buy a block at the deli counter) along with some seasonings until the mixture is smooth; move the pot off the heat; and add shredded extra-sharp cheddar. Residual heat from the sauce will gently melt the cheese without any risk of it breaking.