Preheat the oven to 350℉ and butter and 8x8-inch pan. In a large saucepan over medium heat, melt the ¼ cup unsalted butter. Whisk in the ¼ cup sweet white rice flour or all-purpose flour or all-purpose 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 cup shredded cheddar cheese and ¼ cup shredded parmesan cheese until melted. Stir in the ½ teaspoon fine sea salt and ¼ teaspoon black pepper.
While sauce is cooking, make the ½ pound elbow macaroni, then drain and set aside. Mix the 4 tablespoons unsalted butter with ¼ cup breadcrumbs.
To assemble, fold the cooked pasta into the sauce, then add half of the mixture to the prepared baking dish. Sprinkle ½ cup shredded mozzarella cheese over, then top with the remaining pasta and cheese sauce. Evenly scatter the prepared breadcrumbs over the top.
Place in the oven and bake for about 20 minutes, until the cheese sauce is bubbling and the breadcrumbs are toasted. Let stand 5-10 minutes out of the oven before serving.
Notes
Don't overcook the pasta. Follow the instructions on your box of pasta, then cook for about 1-2 minutes less time so that the pasta is al dente (slightly firm). It will soften a bit more as it bakes and soaks up the cheese sauce.
Rest. Allow the baked mac and cheese to rest for about 5-10 minutes after it comes out of the oven so that it firms up a bit.
Try different pasta shapes. We love elbow macaroni for that classic mac and cheese look, but you can use any kind of pasta shape you have on hand. Use a "nooks and crannies" type of pasta, like penne or ziti, so that it holds the cheese sauce.
Gluten-free breadcrumbs. To keep this gluten-free, use ¼ cup of gluten-free breadcrumbs rather than regular.