In a medium saucepan melt the butter over medium-low heat.
2 tbsp unsalted butter
Sprinkle in cornstarch and then whisk gently for ~3 minutes or until the cornstarch and butter have taken on a caramel color.
2 tbsp cornstarch
Slowly pour in the warmed milk (about 1/2 cup at a time), garlic powder, and salt, whisking constantly. Cook for 2 minutes while whisking.
1 1/2 cup whole milk, 1/2 tsp garlic powder, 1/4 tsp sea salt
Add the extra-sharp cheddar and Parmesan and stir gently on low heat until melted. This can take 3-5 minutes.Pro tip: the Parmesan will be the last to melt because it is a harder cheese.
1 1/4 cup extra sharp cheddar, 1/4 cup Parmesan
Salt and pepper to taste.
Notes
Do not skip the step of warming the milk before adding it to the gluten-free roux. This will prevent the sauce from curdling/breaking.
Whenever possible, grate your own cheese for the smoothest homemade cheese sauce.
You can keep this sauce covered in the fridge for 2-3 days. To reheat: cook over low heat, stirring slowly with a whisk to make sure the sauce doesn’t curdle or break. Or microwave in 15-second increments, stirring each time.
Cornstarch alternatives: arrowroot powder, tapioca starch, or sweet rice flour. A cup for cup gluten-free flour (like King Arthur) will also work.
Using 1/2-1 tsp of dry mustard powder and/or chicken bouillon powder can also intensify the taste of the sauce.