Preheat the oven to 325 degrees F and lightly grease an 8' square (or similar size) baking dish.: The oven will give you a gentle, even bake that melts the top without drying the interior. You should feel a mild warmth from the oven as it reaches temperature, and the light greasing prevents sticking while adding a bit of buttery aroma. Doing this step ahead keeps you from scrambling later, which helps maintain smooth assembly. A common misstep is preheating to a much higher temperature, which can cause the top to brown too quickly while the interior remains underheated.
Cook the macaroni to al dente, according to package instructions. Drain and set aside.: The pasta should be firm to the bite yet tender, offering a slight resistance when you chew. While boiling you will notice the water roiling and a faint starch scent; when you taste the pasta it should not be chalky. Drain promptly to stop cooking and avoid a waterlogged bake. Overcooking here results in a mushy texture after baking, and undercooking will make the macaroni feel gummy in the finished dish.
Make the sauce: Melt the butter in a medium saucepan over medium heat. Blend in the flour, salt, and pepper. Cook for 2 minutes. Slowly add the milk and half and half, stirring constantly, and cook over medium-low heat for a few minutes until the mixture has thickened like the consistency of gravy, about 5-7 minutes. Remove from heat and stir in 1 cup shredded cheese, stirring just until melted.: Melt the butter in a medium saucepan over medium heat. Blend in the flour, salt, and pepper. Cook for 2 minutes. Slowly add the milk and half and half, stirring constantly, and cook over medium-low heat for a few minutes until the mixture has thickened like the consistency of gravy, about 5 to 7 minutes. Remove from heat and stir in 1 cup shredded cheese, stirring just until melted. : As the butter melts you will hear a soft sizzle and see a glossy pool form, then the flour will bloom into a fragrant, toasty base. When you add the dairy gradually you will notice the sauce thicken and develop a velvety sheen; it should coat the back of a spoon. Stirring constantly prevents lumps and helps the starches hydrate evenly, producing a cohesive texture. If the heat is too high the milk can scald, creating off flavors, so keep the temperature controlled and remove from heat before adding most of the cheese. A typical error is adding all the cheese while the sauce is too hot, which can cause separation or a grainy texture.
Combine: Add the cooked macaroni noodles and toss to coat. Pour half of the pasta mixture into the prepared baking dish. Sprinkle 1/2 cup cheese over the top. Add remaining pasta and sprinkle with remaining cheese.: Add the cooked macaroni noodles and toss to coat. Pour half of the pasta mixture into the prepared baking dish. Sprinkle 1/2 cup cheese over the top. Add remaining pasta and sprinkle with remaining cheese. : When you fold the macaroni into the sauce you should see glossy strands and pockets of rich cream; each noodle should be evenly coated. Layering the pasta and cheese ensures pockets of melted cheese on top and in the interior, creating contrast between gooey and slightly baked textures. Press gently so the sauce seeps into the pasta crevices. Avoid packing the dish too tightly, as dense layers can prevent even heating during the short bake.
Bake for 15-20 minutes or until cheese is melted on top.: In the oven the surface will shift from glossy to a gentle bubbling and the cheese will become uniformly melted, emitting a warm, savory aroma. Look for small, golden spots and a consistent melt across the top to know it is ready. Let the dish rest for a couple of minutes after removing it, because the sauce will set slightly and be easier to portion. A frequent mistake is leaving it in longer than necessary, which can dry the sauce and reduce creaminess.