Directions:

Bring a large pot of salted water to a boil. Meanwhile, melt the butter in a medium skillet over medium heat. Once the foam subsides, start stirring with a wooden spoon while scraping up any bits on the bottom of the pan. Continue cooking until the bits are almond brown in color. Remove from the heat.
Tilt the skillet to scoop out 4 tablespoons brown butter into a large saucepan. Add the bread crumbs to the remaining butter left in the skillet and set over low heat. Sprinkle with salt and stir well. Toast, stirring occasionally, until the crumbs are crisp and golden brown, 3 to 7 minutes. Remove from the heat.
Drop the pasta into the boiling water and cook, stirring occasionally, until al dente as directed on the package.
While the pasta cooks, add the flour to the brown butter in the saucepan and set over medium heat. Cook, whisking continuously, until the raw flour smell dissipates and the mixture is bubbling, about 2 minutes. While whisking, add the milk ¼ cup at a time in a thin stream. Keep whisking the whole time to ensure the sauce stays very smooth. Once all the milk is added, keep whisking until the sauce comes to a boil. Boil for 2 minutes while whisking. Remove from the heat and add the cheese. Let stand for 30 seconds, then stir with a rubber spatula or wooden spoon until smooth.
Reserve 1 cup pasta cooking water, then drain the pasta. Immediately pour it into the cheese sauce and fold until evenly coated. The sauce will stiffen as it cools, so you want it runny at this stage; if it is too thick, add the reserved water a tablespoon at a time to loosen it to that classic boxed mac and cheese consistency. Season to taste with salt and pepper. Transfer to a serving dish and top with the crumbs. Serve immediately.