Cook the pasta short: Bring a large pot of salted water to a boil and cook the elbow macaroni 2 minutes less than the package directions, so it's still firm in the center. Drain and set aside.

Make the roux: Melt the 1/2 cup butter in a large pot or Dutch oven over medium heat. Whisk in the flour and keep whisking for 1 minute, until the mixture looks like wet sand and smells slightly nutty.

Build the sauce: Pour in about 1 cup of the warm milk and whisk hard until it's smooth, then add the rest in a steady stream. Cook for 5-6 minutes, whisking often, until the sauce coats the back of a spoon and leaves a clear line when you swipe a finger through it.

Season and melt in the cheese: Stir in the salt, pepper, mustard powder, garlic powder, and cayenne if using. Turn the heat to low and add 3 cups of the cheddar and all the gruyere a handful at a time, stirring until each addition disappears before adding more.

Combine: Add the drained macaroni to the pot and fold until every noodle is coated. Spread it into a greased 9x13 baking dish and scatter the remaining 1 cup of cheddar over the top.

Add the panko top: Toss the panko with the 2 tablespoons melted butter in a small bowl until the crumbs look damp, then sprinkle them evenly over the cheese.

Bake: Bake at 350°F for 20-25 minutes, until the edges bubble and the panko is deep golden. Let it stand 5 minutes before scooping so the sauce sets up.