Boil a large pot of salted water. Cook pasta according to package instructions.
In a saucepan over medium heat, melt the butter. Whisk in flour and cook until golden and bubbling.
Pour in the half and half, whisking continuously until smooth. Add mustard powder, garlic powder, black pepper, nutmeg, cayenne (if using), and salt. Mix well.
Reduce heat, add all cheeses, and whisk until smooth.
Drain cooked pasta and add it to the sauce. Stir well and serve warm! Enjoy your homemade Panera-style Mac & Cheese!