Add sour cream, cream of chicken soup, and 1/2 cup melted butter. Mix well.
Add salt, onion powder, and shredded cheddar cheese. Continue mixing until evenly combined.
Spread mixture into a greased 9x13 baking dish.
Toss crushed cornflakes with the remaining 1/2 cup melted butter.
Sprinkle buttered cornflakes evenly over the casserole.
Bake uncovered at 350°F for 45 to 50 minutes, until hot, bubbly, and golden on top.
Notes
Thaw hashbrowns fully before mixing so the casserole does not turn watery. Add the cornflake topping right before baking if making ahead. If the topping browns too quickly, lay foil loosely over the pan without sealing it tight. Leftovers keep refrigerated for up to 4 days and reheat best in the oven.