Set your oven to 350°F and bring 3 quarts of water to a rolling boil in a large pot. Add the macaroni and 1 tablespoon of kosher salt to the boiling water and cook until the pasta is tender but still has a bite, about 6 minutes, stirring occasionally. Reserve 2 cups of the pasta cooking water before draining the macaroni; then return it to the pot and cover to keep warm.

Butter a 13 by 9-inch baking dish generously and set aside. In a bowl, combine the fresh breadcrumbs, Parmesan cheese, and half of the cooked crumbled bacon for the topping and put it aside. In a small bowl, whisk together the flour, black pepper, dry mustard, and the remaining 1 1/2 teaspoons kosher salt. In a medium saucepan, gently warm the whole milk and buttermilk over medium heat without stirring, until it just starts steaming but does not boil, about 4 to 5 minutes. Remove from heat.

In a large heavy saucepan over medium-high heat, melt the butter. Whisk in the flour mixture and keep stirring until the blend thickens, looks smooth, and has a golden hue with a toasted scent, about 2 minutes.

Gradually pour in the warm milk mixture while whisking continuously. Increase the heat to high and bring to a boil. Stir frequently until the sauce thickens and coats the back of a spoon, about 3 minutes.

Remove the pan from heat and add the shredded Cheddar and Monterey Jack cheeses. Stir in the beaten eggs until you have a silky, uniform cheese sauce.

Uncover the macaroni and stir it well; if it seems clumpy, mix in a little of the reserved warm pasta water and drain again if necessary. Add the cheese sauce and the remaining crumbled bacon to the pasta and stir gently to combine everything evenly.

Pour the mixture into the prepared baking dish and sprinkle the breadcrumb topping evenly over the surface.

Bake in the preheated oven for 35 to 40 minutes, or until the mac and cheese is set, puffed up, and has a lightly browned, crispy top. Let it rest slightly, then serve warm for the best comfort food experience.