In a saucepan, melt the butter over medium heat.
Add the flour and whisk continuously for about 1 minute to create a roux.
Gradually pour in the milk while whisking to avoid lumps.
Continue to cook, stirring frequently, until the mixture thickens (about 3-5 minutes).
Add the shredded cheese to the thickened milk mixture.
Stir until the cheese is fully melted and the sauce is smooth.
Season with salt, pepper, and optional garlic powder and mustard.
If the sauce is too thick, add a little more milk to reach the desired consistency.
Remove from heat and serve immediately over your favorite dishes.
Store any leftovers in an airtight container in the fridge.