01 - In a medium saucepan, melt butter over medium heat.
02 - Whisk in flour and cook, stirring constantly, for 1 to 2 minutes until foamy but not browned.
03 - Slowly pour in milk while whisking constantly to prevent lumps.
04 - Continue to cook, stirring until mixture thickens and gently bubbles, about 3 to 4 minutes.
05 - Reduce heat to low and add cheddar and Parmesan cheeses gradually, stirring until melted and smooth.
06 - Add salt, pepper, ground mustard, and cayenne pepper if using; adjust seasoning to taste.
07 - Serve warm over vegetables, pasta, or use as a dip.