This is the classic crowd-pleaser, a little richer and a lot smoother. Made with a generous amount of Vemont cheddar, this dish is the perfect compliment to a cold day and sure to warm the soul. The extra-sharp cheddar and dash of mustard might remind you of Welsh rarebit.
Preheat the oven to 350°F. Butter a 13x9-inch baking dish or 2-½-quart casserole. Melt 3 tablespoons of the butter over moderate heat in a large, preferably nonstick, saucepan. Add the flour and cook gently for 1 minute, stirring constantly. Increase the heat slightly, then add the milk, 1 cup at a time, whisking constantly as the sauce thickens. Whisk in the heavy cream. Stir in the cheddar cheese, about half at a time, and reduce the heat so that the sauce does not boil. Remove the pan from the heat and stir in the salt, mustard, and cayenne, if using, until smooth and evenly blended. Pour the sauce over the macaroni and stir well. Transfer the mixture to the prepared baking dish and smooth the top. Mix the bread crumbs and Parmesan cheese in a small bowl and then spread it evenly over the macaroni. Dot with the remaining 2 tablespoons of butter. Bake for 25 minutes, or until bubbly.