Set an oven rack 6 inches from the heat source and preheat the oven to 220°C.
Bring a large pot of salted water to a boil. Add the macaroni and cook until it is al dente, about 6 minutes. Strain, reserving 1 3/4 cups of the pasta water. Set aside.
Heat the half-and-half and cayenne in a large saucepan over medium heat until it just comes to a simmer. Continue simmering until the half-and-half is reduced to 1 1/2 cups, about 15 minutes. Add the cream cheese and stir until melted. Whisk in the Cheddar and Gruyere until all the cheese is melted and the sauce is smooth.
Add the macaroni and the reserved pasta water to the saucepan and stir to combine. (It will look very loose, but the pasta will suck up a lot of the sauce as it bakes.) Season to taste with salt if needed. Pour into a 13-by-9-inch baking dish. Toss the bread crumbs with the butter and a sprinkle of salt in a small bowl and sprinkle evenly over the top of the macaroni and cheese. Bake until the cheese is bubbly and the bread crumbs are lightly browned and crispy, about 10 minutes.
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