Make the sauce: In a large saucepan over medium heat, melt the 2 tablespoons unsalted butter. Whisk in the 2 tablespoons sweet white rice flour and cook for about 2 minutes, whisking, until a paste forms. Whisk in the ½ cup whole milk, whisking constantly, until smooth, then whisk in the 1 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 cup shredded cheese until melted. Stir in the ¼ teaspoon fine sea salt and ¼ teaspoon black pepper.
Meanwhile, boil a pot of salted water and cook the 1 box gluten free elbows to the instructions. Drain, then fold the cooked pasta into the sauce. Taste for salt and pepper, then serve,.
Notes
Don't overcook the pasta. You don't necessarily have to cook the pasta to al dente, but watch so that it doesn't overcook and get mushy. Gluten-free pasta is a bit more finicky than regular, so sometimes I'll slightly undercook it so that it doesn't disintegrate when it soaks up the cheese sauce.
Make it dairy-free - Use non-dairy butter or margarine for the butter and non-dairy milk and cheese.
Add veggies - A nice add-in here is baby spinach - just fold it into the pasta when combined with the cheese sauce. Frozen broccoli is another option - just add 1-2 cups into the mix before baking. Or you could saute veggies like chopped red onion, bell pepper, mushrooms, asparagus, etc., and stir them into the pasta and cheese sauce.
Try different pasta shapes. While macaroni or elbows are the classic mac and cheese shape, you can use any kind of pasta shape. I prefer tubed or shaped "nooks and crannies" pasta like penne, ziti, or macaroni, so that the cheese sauce really gets in there.
Different cheeses. I love a combo of sharp cheddar and Parmesan, but feel free to use others. Nice options include Gruyere, Swiss, pepper or regular Jack, mild cheddar, or Fontina cheese.