Bring a large pot of salted water to a boil. Add the pasta and cook it just until al dente, according to package instructions. Drain the pasta set aside.: The first sign you are on track is the water at a rolling boil, sending up large, lively bubbles and a faint steam. When you add the macaroni , the pot will quiet for a moment before the surface action resumes. As the pasta cooks, you will see the shapes swell and the water cloud slightly with starch. Taste a piece near the lower end of the package time to confirm it is al dente, tender on the outside but with a slight bite in the center. Drain immediately when done, and give it a quick shake in the colander to remove excess water, because watery pasta can thin the sauce. A common mistake is overcooking leading to mushy texture, so set a timer and taste early if unsure.
While the pasta is cooking, melt the butter over medium heat in a large pot. Turn the heat to low and whisk in the flour and ground mustard to create a paste. Whisk in milk until smooth. Turn the heat to medium and whisk until the sauce starts to thicken, about 2 minutes.: You will hear a gentle sizzle as the unsalted butter softens and then becomes glossy and slightly foamy. Keep the heat steady so the butter melts evenly without browning, which would add a nutty note you do not want here. Use a spatula to move it around the pot so it melts uniformly. If the butter starts to brown, lower the heat immediately because the roux needs a pale, neutral base. Browned butter can shift the sauce flavor unexpectedly.
Stir in the shredded cheese until the cheese is melted and the sauce is smooth.: As you add the all purpose flour to the melted butter , whisk constantly to form a smooth paste, watching for the raw flour smell to mellow. The texture should be like a thick, tacky paste that clings to the whisk rather than a loose batter. Add the ground mustard now so it toasts slightly and releases its scent, which brightens the future sauce. If lumps form, keep whisking on low heat; the goal is a cohesion that will absorb the milk without clumping. The frequent issue at this point is rushing, which leads to a lumpy base, so take your time and keep the whisk moving.
Add the drained pasta and stir until well coated with the cheese sauce. Season with salt and black pepper, to taste. Garnish with a little shredded cheese and crushed saltine crackers, if desired. Serve immediately.: Pour the room temperature milk in gradually while whisking energetically so the roux accepts it seamlessly and the mixture becomes glossy. As the sauce warms, you will notice it shift from thin to velvety, leaving a soft trail on the whisk. Bring it up to medium heat and keep whisking; within about 2 minutes you should feel the sauce gain body, coating the back of a spoon. The aroma will become rich and inviting, and you should see small steam wisps. If the sauce boils hard, it can break or scorch, so maintain a gentle simmer. A typical mistake is pouring in cold milk too quickly, which can create lumps.
Stir in the shredded cheese until the cheese is melted and the sauce is smooth.: As you add the shredded sharp cheddar cheese in handfuls and stir, the sauce will go from glossy white to a warm, golden hue, and you will hear a soft settling sound as the cheese melts. Keep the heat low to medium so the cheese melds without seizing up or becoming grainy. Stir patiently until you achieve a homogenous, silky texture. If the sauce looks oily or separated, remove it from direct heat and whisk in a splash of warm milk to bring it back together. Overheating is the main culprit for a grainy cheese sauce, so watch the temperature closely.
Add the drained pasta and stir until well coated with the cheese sauce.: When you fold the drained macaroni into the sauce, listen for a gentle coating sound and watch as each elbow becomes glossy and enveloped. The visual cue is a uniform sheen over the pasta without puddles of thin liquid at the bottom of the pot. Taste and season with kosher salt and freshly ground black pepper sparingly until the balance feels right. If the sauce seems too thick, stir in a little warm milk to reach the texture you prefer. A common oversight is under seasoning, so taste before serving.
Season with salt and black pepper, to taste. Garnish with a little shredded cheese and crushed saltine crackers, if desired. Serve immediately.: The finishing touches elevate the dish visually and texturally. Freshly ground black pepper adds a warm aromatic lift, while extra shredded cheese melts on top and crushed saltine crackers give a delightful crunch. Serve right away while the sauce is still creamy and warm, because the texture changes as it cools. The biggest mistake here is letting it sit too long, which can cause the sauce to thicken and clump; if that happens, rewarm gently with a splash of milk and stir until smooth.