Preheat oven to 175°C (350°F). Grease a 9x13 inch baking dish with butter or non-stick spray.
Spread thawed hashbrowns on a clean kitchen towel and press firmly to wring out as much moisture as possible.
In a large bowl, combine the dried hashbrowns, cream of chicken soup, sour cream, 170g cheddar, melted butter, diced onion, garlic powder, onion powder, salt, and pepper. Mix until evenly combined.
Transfer the mixture to the prepared baking dish and spread into a flat, even layer.
In a small bowl, toss crushed cornflakes with 60g of melted butter until fully coated.
Spread buttered cornflakes evenly over the potato layer. Scatter the remaining 55g of shredded cheddar on top.
Bake uncovered at 175°C (350°F) for 40 to 45 minutes, until the crust is deep golden brown and the edges are actively bubbling. Center should reach 74°C (165°F) on an instant-read thermometer.
Let the casserole rest for 5 minutes before scooping and serving.