Preheat oven to 375°F (190°C). Grease a 12-cup muffin tin.
Cook macaroni according to package instructions; drain and set aside.
In a saucepan, melt butter over medium heat. Stir in flour to form a roux.
Slowly whisk in milk until smooth and thickened.
Add cheddar, mozzarella, and Parmesan cheese; stir until melted.
Season with mustard powder, garlic powder, onion powder, salt, and pepper.
Allow the mixture to cool slightly, then stir in the beaten egg.
Combine cooked macaroni with the cheese sauce.
Spoon mixture into muffin cups and sprinkle breadcrumbs on top if using.
Bake for 18–25 minutes, or until golden and set.
Cool in tin for 10 minutes before removing.