In a medium saucepan, melt the butter over medium heat.
Add the flour and whisk constantly for about 1 minute to form a roux.
Slowly pour in the milk, whisking constantly to avoid lumps.
Gradually whisk in the Guinness beer until combined.
Cook for several minutes, stirring often, until the mixture thickens slightly.
Reduce heat to low, and gradually add the shredded cheddar cheese, stirring until melted and smooth.
Add the garlic powder, onion powder, and mustard powder. Stir well to combine.
Season with salt and pepper to taste, then remove from heat.
Serve immediately with pretzels, bread, or your favorite dippers.