01 - Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for 1 minute, stirring constantly, until roux forms and bubbles gently.
02 - Slowly pour milk into the roux while whisking continuously to prevent lumps. Bring to a gentle simmer and cook for 2-3 minutes until sauce thickens enough to coat the back of a spoon.
03 - Reduce heat to low. Gradually add cheddar and mozzarella cheeses, stirring constantly until completely melted and sauce becomes smooth and glossy.
04 - Stir in garlic powder, onion powder, smoked paprika, black pepper, and salt. Taste and adjust seasoning as desired. Serve immediately over crispy fries or as a dipping sauce.