Melt 2 tablespoons butter in a saucepan over medium heat
Whisk in 2 tablespoons all-purpose flour
Cook for 1 minute, whisking constantly
Slowly whisk in 2 cups milk
Cook until the mixture thickens, about 3 to 5 minutes
Reduce heat to low
Add 2 cups shredded cheese, a little at a time
Stir until the cheese is fully melted and smooth
Season with salt and pepper
Add a pinch of mustard powder or paprika if desired
Use immediately for mac and cheese