Tear sandwich bread into large pieces and place in a food processor. Pulse bread 10-15 seconds or until it reaches a fine crumb. 5 slices should make around 2 cups of breadcrumbs.
Using a handheld cheese grater, grate 3 tablespoons of cold butter into breadcrumbs, toss to combine.
In a large stock pot or Dutch oven, boil macaroni in salted water until al dente, 8-9 minutes. Drain pasta, return pasta to pot it was cooked in and set aside.
While the pasta is cooking prepare the cheese sauce, in a large saucepan over medium heat, melt the 5 tablespoons of butter. Whisk in flour, salt, pepper, and cayenne pepper. Cook roux for 3-4 minutes, whisking regularly.
Whisk in milk and cream into roux mixture, continue to cook sauce, whisking regularly, until it begins to thicken, 4-5 minutes.
Whisk in grated cheese, stir until all cheese is melted and fully incorporated into the sauce, 4-5 minutes.
Pour cheese sauce over cooked macaroni, stir until well mixed.
Pour mixture into a 9x13 inch baking dish, top with breadcrumb topping.
Bake macaroni and cheese, uncovered, for 20 minutes, or until bubbly and topping has browned.