01 - Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for 1 minute, stirring constantly to form a smooth roux.
02 - Slowly pour in milk while whisking continuously until smooth. Bring to a gentle simmer and cook for 2–3 minutes until slightly thickened.
03 - Reduce heat to low. Add cheddar and mozzarella cheeses, stirring until completely melted and the sauce is smooth and creamy.
04 - Stir in garlic powder, onion powder, smoked paprika, black pepper, and salt. Taste and adjust seasoning as desired.
05 - Serve warm over crispy fries or use as a dipping sauce. The sauce is best enjoyed immediately while hot and smooth.