Bring a large pot of salted water to a boil and cook the macaroni until al dente. Drain and set aside.
Shred all cheese from blocks and divide for sauce and topping.
Melt butter in a pan, whisk in flour, and cook 1–2 minutes to form the roux.
Slowly whisk in milk and cream; simmer until thickened. Season with salt, pepper, and optional nutmeg or mustard.
Remove from heat and add cheeses gradually, stirring until smooth and silky.
Fold in the cooked macaroni, adding a splash of milk if the sauce thickens too much.
Grease a baking dish and layer mac and cheese with extra shredded cheese in the center.
Mix breadcrumbs with melted butter or oil; sprinkle over the top (or use cheese only).
Bake at 350–375°F for 25–30 minutes until golden, bubbly, and slightly crisp around the edges.
Rest 5 minutes before serving so the sauce sets and stays creamy.