Preheat oven to 350°F (175°C). Grease a 9×13-inch baking dish or a similar ovenproof casserole.

Bring a large pot of salted water to a boil. Add macaroni and cook for 1 to 2 minutes less than package directions so the pasta is very al dente. Drain and set aside.

In a medium saucepan over medium heat, melt 4 tablespoons butter. Sprinkle in the flour and whisk continuously for 1 to 2 minutes until the mixture is bubbly and pale golden. Do not let it darken.

Gradually whisk in the warmed milk and cream. Keep whisking to remove lumps. Cook for 4 to 6 minutes, stirring frequently, until the sauce thickens and coats the back of a spoon.

Remove sauce from heat and stir in salt, pepper, mustard powder, and smoked paprika. Add the shredded cheddar, Gruyère, and mozzarella in batches, stirring until each batch melts and the sauce is silky.

Taste and adjust seasoning.

Fold the drained pasta into the cheese sauce until evenly coated.

In a small bowl mix panko, melted butter, Parmesan, and a pinch of salt and pepper until crumbs are evenly coated.

Transfer the sauced pasta to the prepared baking dish. Sprinkle the breadcrumb topping in an even layer. Bake uncovered for 20 to 25 minutes until the top is golden and the sauce is bubbling around the edges.

For extra browning, place under the broiler for 1 to 2 minutes watching carefully. Let rest 5 minutes before serving.