Preheat your oven to 350°F (175°C).
Boil a large pot of salted water and cook the elbow macaroni until al dente.
Drain the macaroni and set aside.
In a saucepan, melt the butter over medium heat.
Whisk in the flour and mustard powder, cooking for about 1 minute.
Gradually add milk and heavy cream, whisking constantly until thickened.
Stir in 1.5 cups of shredded cheese until melted and smooth.
Combine the cheese sauce with the cooked macaroni, mixing well.
Transfer the mixture into a greased baking dish.
Top with remaining cheese and breadcrumbs (if using), then bake for 20 minutes.