Move an oven rack to the middle of the oven and preheat to 400° F/200° C.
In the small bowl, melt 2 tablespoons of butter and stir together with the fresh breadcrumbs. Set aside.
In the pot, bring 4 quarts of water to a boil. Add 1 tablespoon of kosher salt and stir in the pasta. Cook until the pasta is almost tender but still firm to the bite. Don’t overcook because it still needs to go into the oven with the cheese sauce. Drain the pasta and keep it in the colander.
Melt the remaining 6 tablespoons of butter in the hot pot over medium heat. Add the garlic, mustard, and cayenne and cook until fragrant, about 30 seconds.
Add the flour and whisk constantly for about 1 minute. You want the flour to take on a golden hue and to give off a slightly nutty aroma. Add the broth slowly while whisking until the mixture is combined, then whisk in the milk. Bring to a simmer and continue to cook while whisking for about 6 minutes. The mixture should thicken just a bit.
Remove from the heat and stir in the Colby and cheddar until they are completely melted. Add the pepper and a little more salt to taste, then stir in the cooked macaroni. Break up any clumps and make sure the macaroni is completely coated with the sauce.
Assemble and Bake
Pour the mixture into the prepared baking dish and sprinkle the breadcrumbs over the top. Bake in the preheated oven for 25 to 30 minutes, until the edges are bubbling and the top is golden brown. Removed from the oven and allow to stand and cool for about 10 minutes before serving.