Melt the butter in a medium saucepan over medium heat. Once melted, add the flour and whisk continually until dissolved. Continue whisking for one minute to cook off the flour flavor.

Add the garlic powder, salt, pepper, and cayenne (if using) to the roux. Slowly pour in the milk, whisking continually to prevent lumps.

Increase the heat to medium-high and continue whisking until the mixture comes to a low simmer (do not boil). Continue to whisk until the mixture has thickened.

Add the shredded Cheddar cheese and whisk until it's completely melted and smooth.

Stir in the onion powder. Taste and add more salt and pepper as needed. Serve immediately.

This sauce keeps in the fridge in a sealed container for up to 3 days. Reheat on the stovetop over low heat, stirring frequently, adding a little milk if too thick.