In a large pot, bring salted water to a boil. Add elbow macaroni and cook according to package instructions until al dente. Drain and set aside.

In a saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute.

Gradually whisk in milk, stirring constantly until the mixture thickens, about 5 minutes.

Remove from heat and stir in cheddar cheese, mozzarella cheese, salt, pepper, and mustard powder until smooth.

Preheat oven to 350°F (175°C). In a large bowl, combine cooked pasta and cheese sauce. Mix well.

Transfer to a greased baking dish. In a small bowl, mix bread crumbs with melted butter and sprinkle over the top.

Bake in the preheated oven for 20 minutes, or until the top is golden brown.