01 - In a medium saucepan, melt the butter over medium heat.
02 - Whisk in the flour and cook, stirring constantly, for 1 to 2 minutes until foamy but not browned.
03 - Slowly pour in the milk while whisking constantly to prevent lumps.
04 - Continue stirring and cooking until the 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 - Season with salt, pepper, ground mustard, and cayenne pepper if desired. Adjust seasoning to taste.
07 - Serve warm over vegetables, pasta, or as a dip.