Preheat oven to 400 degrees F.: As the oven warms you should notice a faint hum and then warmth radiating when you open the door to check. A fully preheated oven ensures the cheeses start melting quickly and the top gains light color instead of taking a long time to warm through. If you skip preheating the top may not brown evenly, so always allow the oven to reach temperature before placing the dish inside.
Bring a large pot of water to a boil over high heat.: You will see vigorous bubbles and hear rolling water as it comes to a boil. Salting the water is important because it seasons the elbow macaroni from the inside out. If the boil is not strong enough the pasta can become gummy, so be patient and wait for a true, rolling boil before adding pasta.
When the water is boiling add in macaroni and cook until al dente.: You will feel the pasta giving slightly when you bite into it, a slight resistance that tells you it is al dente. This texture matters because the pasta will finish cooking in the oven and you want it to hold shape, not become mushy. Overcooking here is a common mistake, and it leads to a softer mouthfeel once baked.
Drain and set aside until needed.: After pouring the pasta into a colander you should see steam rising and the pasta glistening with a thin film of water. Let it rest briefly so excess moisture evaporates, otherwise the final casserole can be watery. Avoid rinsing the pasta because rinsing removes surface starch that helps cheeses cling to each piece.
In a medium-sized mixing bowl add all the cheese and stir to combine.: As you mix the shredded cheeses together you can already see the color variation and smell a nutty, dairy aroma. Combining them evenly ensures each bite contains a balanced mix. A common error is to heap one cheese in one area, causing uneven melt and flavor, so take a moment to toss them thoroughly.
In a 9×13 dish add butter cubes evenly over the bottom.: When the butter melts it will spread a golden, glossy coating across the dish, which adds flavor and prevents sticking. The butter also helps the bottom layer of pasta develop a hint of toasted richness. If you crowd the butter or place it unevenly you may end up with hot spots, so distribute the cubes deliberately.
Add in half the pasta and cover with half the shredded cheese.: When you place the first layer of elbow macaroni into the dish you will notice steam settling and the starches ready to bond with the cheese. The first layer of cheese begins to soften from the dish warmth and butter beneath. Layering promotes even melt and prevents clumping, whereas dumping everything together can create an uneven texture.
Top with remaining pasta and remaining cheese.: The top layer of cheese will sit visibly on the pasta, creating a surface that browns attractively in the oven. Press lightly so the pasta and cheese make contact, which encourages melting into one cohesive casserole. If the top is too sparse on cheese, the bake will lack that signature gooey top, so be sure to distribute it evenly.
Drizzle milk over the top of the mac and cheese and add to the oven.: The thin stream of milk will settle into crevices, helping the cheeses soften and create pockets of creaminess while baking. You should hear a faint hiss as the milk hits the hot dish, and the oven will quickly take over to meld the ingredients. Adding too much liquid can make the casserole soupy, so stick to the small amount recommended.
Once the cheese has started to melt, about 10 minutes, open the oven and stir the mac and cheese together.: At this point you will see the cheeses loosening and becoming glossy, and the aroma will be rich and tempting. Stirring now helps redistribute the melted cheese so every piece becomes coated. Be careful when opening the oven, steam will escape and the topping may be soft, so use an oven mitt and a sturdy utensil to stir gently. A common misstep is to stir too early while cheeses are still firm, which can tear the pasta and create a clumpy texture.
Bake for another 20 minutes until the top of the mac and cheese is lightly browned.: During this final period you will hear a faint bubbling and see the cheese transform from glossy to slightly golden on top. The edges may darken a touch, providing contrast and a hint of toasted flavor that I find irresistible. Remove when the top has a light brown hue, not when it is deeply dark, to preserve a creamy interior. Overbaking results in a dry casserole, so watch the color and pull it out when it looks inviting and tender inside.