Method:
Bring a large pot of salted water to a boil. Add macaroni and cook until al dente, 10 to 12 minutes. Drain well.
Meanwhile, whisk together milk, flour, mustard and cayenne in a large saucepan until completely blended then cook over medium heat, stirring constantly, until thickened, 6 to 8 minutes. Add cheeses and stir gently until melted, then add drained pasta to sauce. Season with salt and pepper, stir well and serve hot.