Boil the pasta to al dente according to package directions. Drain over a colander and let cool slightly while you make the sauce.: The pot of boiling water should be lively and aromatic, with steam carrying a faint wheaty scent as you add the pasta . You want the shells to have a slight bite, not limp, so taste one a minute or two before the lower bound of the time on the box. The sound of rolling bubbles will ease as the water returns to a boil after adding the pasta, and you will notice the surface moving with steady motion. Draining promptly prevents the pasta from continuing to absorb water and becoming mushy once mixed with the sauce. A common mistake is overcooking; if shells are soft and collapsing, they will become gummy when combined. After draining, let the pasta sit briefly so it is warm but not steaming hot, which helps the sauce cling without causing separation.
Melt the butter in a large pot over medium heat. Whisk in the flour and cook until slightly golden and no longer floury-smelling.: When the butter melts it will foam gently, and you should smell a warm, buttery note without any brown or nutty aroma yet. Add the flour and whisk constantly; the mixture will become a pale paste that loosens as water from the butter and flour combine. After a minute or two a subtle change in color and the disappearance of raw flour aroma indicates the roux is ready. This step builds the sauce body, so patience matters. If the roux browns too much you will introduce an unintended toasty bitterness, so keep the heat steady. A common error is leaving lumps of uncooked flour ; vigorous whisking prevents that and creates a uniformly smooth foundation.
Gradually pour in the milk first, then cream, while whisking continuously.: As you add the room temperature milk , you will hear soft, steady whisking and the roux will loosen into a smooth, pale sauce. The liquid will steam softly but should not boil. Pouring slowly and whisking continuously creates a stable emulsion and prevents lumps from forming. When the cream goes in, the sauce will take on a richer sheen and thicker mouthfeel. The texture should be glossy and slightly thick on the whisk; if you see streaks of floury paste, continue whisking until fully smooth. A frequent mistake is dumping the dairy too fast, which yields a lumpy, uneven sauce that is harder to fix.
Once the mixture starts to bubble and thicken, stir in the salt, dried mustard, and pepper.: The sauce will evolve from thin to a coating consistency, and you will notice it cling to the back of a spoon in a slow sheet. Gentle bubbling indicates the starch has activated, thickening the base. Adding the kosher salt , dry mustard powder , and black pepper at this point ensures the flavors bloom into the heated sauce. Stirring distributes the seasonings and allows you to taste and correct balance. Avoid boiling vigorously, which can break the emulsion; if it begins to separate, lower heat and whisk steadily. Under seasoning is common, so taste carefully after the cheese is folded in, since the cheese adds saltiness.
Take the skillet off the heat. Then, add in the cheese and stir until completely melted and incorporated.: Removing the pot from direct heat ensures the cheese melts smoothly into the warm sauce without becoming grainy. As you add the freshly shredded cheese , the sauce will release a rich, tangy aroma and gradually thicken further while maintaining silkiness. Stirring patiently will help the cheese melt uniformly and create a glossy, cohesive sauce. If you try to force melting over high heat, the cheese can seize and become stringy or gritty; a gentle approach preserves the velvety texture. If small clumps form, remove a bit of sauce into a separate bowl and whisk vigorously, then fold it back in to smooth the texture.
Add the pasta to the skillet and stir to coat. Taste, and add more salt if needed.: When you fold the warm pasta into the sauce, you should hear a soft, satisfying swish as shells get enveloped in glossy, cheesy coating. Visually, each shell will appear slick and well dressed, with pockets filled with sauce. This is when the dish comes together, so adjust seasoning carefully because the cheese may already contribute salt. If the sauce seems too thick, stir in a splash of room temperature milk to loosen it. A common oversight is not tasting at this stage, which can leave the final dish under seasoned, so sample and tweak before serving.