Preheat the oven to 350°F (175°C)
Cook the macaroni until al dente
Drain the macaroni and set aside
Melt butter in a saucepan over medium heat
Whisk in flour and cook for 1 to 2 minutes
Slowly whisk in milk until smooth
Cook until the sauce thickens
Stir in salt, pepper, and mustard powder if desired
Add shredded cheese and stir until melted
Combine the cheese sauce with the cooked macaroni
Transfer the mixture to a greased baking dish
Top with extra cheese or breadcrumbs if desired
Bake for 20 to 30 minutes until bubbly and golden
Let it rest for a few minutes before serving