Preheat oven to 350 degrees F. Coat a 9x13-inch baking dish with non-stick cooking spray; set aside.
In a large bowl combine the soup, shredded cheese and sour cream. Stir in the potatoes, breaking up any frozen chunks. Spread in an even layer in the prepared baking dish. Bake, uncovered, for 1 hour.
In a small bowl, combine the melted butter and crushed cornflakes.
Stir the potato mixture and sprinkle with the corn flakes mixture. Bake, uncovered, for an additional 20 minutes. Any leftovers should be stored in an airtight container in the refrigerator for up to 4 days.
Notes
Condensed Cream of Chicken Soup - You can substitute any other condensed cream soup if you'd like (cream of mushroom soup, cream of potato soup, cream of celery soup, etc).
Sharp Cheddar Cheese - Feel free to sub in mild cheddar, Monterey Jack, Colby jack cheese, or any other favorite.
Sour Cream - I recommend using full-fat sour cream so that the casserole texture isn't runny. You could also substitute full-fat plain Greek yogurt.
Frozen Diced Hash Browns - Frozen hash browns typically come in diced or shredded varieties; for this casserole we use the diced type. They are usually sold plain or also with pepper and onions; you can use whichever one you would like! You use these straight from frozen, you do not need to thaw them.
Make-Ahead - Mix together the soup, cheese, sour cream, and potatoes and spread it in the baking dish, then cover with plastic wrap and refrigerate for up to 1 day. Remove from the fridge about 30 minutes before you plan to bake and remove the plastic wrap.
Storage - Keep leftovers in an airtight container in the refrigerator for 3 to 4 days.
Reheating Instructions - Reheat individual portions in the microwave, or if reheating the entire casserole, cover with foil and heat in a 350-degree oven for 20 to 30 minutes or until heated through.
Freezing Instructions - Prepare the casserole as outlined above in the make-ahead instructions (mix casserole ingredients and spread in your baking pan). Cover with plastic wrap, then cover with a layer of aluminum foil. Freeze for up to 3 months, then thaw in the refrigerator overnight before baking.