Butter a shallow 1½-quart (48–fl oz/1.5-l) baking dish. In a small frying pan over medium-high, melt 1 tablespoon of the butter. Add the bread crumbs and fry until golden, about 4 minutes. Drain on paper towels.
In a frying pan over medium heat, fry the bacon until cooked but not crisp, 3–5 minutes. Drain on paper towels. In a saucepan over medium-high heat, melt the remaining 3 tablespoons butter. When the butter foams, whisk in the flour, ½ teaspoon salt, ¼ teaspoon black pepper, and the cayenne until a paste forms. Slowly whisk in the hot milk, reduce the heat to medium, and cook, whisking constantly, until thickened, about 15 minutes. Add three-fourths of the farmstead Cheddar and half of the white Cheddar and stir until melted, 1–2 minutes. Remove from the heat.
Preheat the oven to 375°F (190°C). Bring a large pot of salted water to a boil over high heat. Add the macaroni, stir well, and cook until the pasta is al dente, according to package directions. Drain, add to the prepared baking dish, and toss with the bacon. Pour in the cheese sauce and toss to mix well. Top with all the remaining cheese and then the bread crumbs. Bake until the sauce is bubbling and the top is golden, about 30 minutes. Let stand for a few minutes before serving.