In a large saucepan over medium heat, melt the ¼ cup unsalted butter. Whisk in the ¼ cup sweet white rice flour and cook for about 2 minutes, whisking, until a paste forms. Whisk in the 1 cup whole milk, whisking constantly, until smooth, then whisk in the 1 ½ cups 2% milk until a smooth sauce forms. Cook, whisking often, until a thick sauce forms, about 5 minutes.
Remove the sauce from the heat and stir in the 1 ½ cups shredded cheese until melted. Stir in the ½ teaspoon fine sea salt and ¼ teaspoon black pepper.
Notes
What kind of cheese to use? Almost any cheese will work here! As mentioned, I love sharp cheddar cheese and parmesan for bold flavor and salt. But a medium or mild cheddar is also great, especially when cooking for younger palates. Other nice options include Gruyere, pepper or regular Jack cheese, Swiss, Gouda, and Fontina.
Cook the roux fully. After whisking in the sweet white rice flour, make sure to cook the mixture for the full two minutes. It should be slightly browned. This ensures you won't taste any flour in the finished sauce.
Keep cooking the sauce. Note that the 5 minutes to thicken the sauce in the recipe directions is a baseline. Sometimes it takes a bit longer for the sauce to fully thicken. This is fine! You want it to be pretty thick, so keep cooking until it is pourable but not liquid.
Melt the cheese completely. When adding the shredded cheese, keep stirring until it fully melts into the sauce. Meaning: you will not see any bits of cheese in the sauce!
Double up. This recipe increases nicely. For a larger portion of cheese sauce, double the ingredients.
Can I make it ahead? Yes, this is a great sauce to make ahead. You can store it in the fridge for a few days, then either reheat or stir it into cooked pasta for a quick pot of mac and cheese.