Melt butter in a sauce pan. Add flour, onion powder, garlic powder and cayenne pepper over medium high heat.Cook for 1 minute. Stir in beer and milk a little bit at a time, whisk until smooth after each addition.keep cooking until thick and bubbly over medium high heat. Reduce heat to low and stir in cheese slowly, until just melted and smooth. Add mustard (if using) and salt and pepper to taste.
Serve warm with veggies or soft pretzels, or over pasta for the world's best mac 'n cheese.
reheat over low heat stirring often to avoid it going grainy.
