11. Melt butter in a heavy-bottomed saucepan over medium heat until bubbling, about 2 minutes.
22. Whisk in flour quickly, creating a smooth roux. Cook, stirring constantly, until mixture turns pale golden and smells nutty, about 3-4 minutes.
33. Gradually pour in milk, whisking vigorously to prevent lumps. Continue stirring until mixture thickens and begins to simmer, approximately 5-6 minutes.
44. Reduce heat to low. Add grated cheeses in three separate additions, stirring between each to create a silky smooth texture.
55. Incorporate salt, white pepper, mustard powder, and cayenne. Whisk until completely integrated and sauce is glossy.
66. Remove from heat. Let stand 2-3 minutes to allow flavors to develop.