Bring a large pot of salted water to a boil and add the macaroni. Cook for 7 minutes, stirring occasionally, until the pasta is just shy of al dente. Drain the pasta thoroughly and return it to the pot you used to boil it.

Add the unsalted butter to the warm pasta, tossing gently until the noodles are well-coated. Let the macaroni cool for about 5 minutes.

In a small bowl, combine the panko breadcrumbs with melted butter and a pinch of salt. Mix well and set aside.

In a medium saucepan over medium heat, melt 2 tablespoons of butter. Whisk in the flour and cook for about 1 minute until bubbly. Gradually add warmed milk, whisking until the mixture thickens, about 3-4 minutes. Stir in the Gruyere and mozzarella cheeses until melted and smooth.

Pour the cheese sauce over the macaroni, stirring gently to ensure every piece is coated. Transfer to a greased baking dish.

Preheat your oven to 180°C (350°F). Sprinkle the breadcrumb topping evenly over the macaroni and cheese. Bake for about 25 minutes, until the top is golden brown.

Allow the dish to rest for 5-10 minutes before serving.