Thaw the frozen hash browns in the refrigerator for 8 to 12 hours, then tip them into a colander to drain and blot dry with a clean towel or paper towels. Break up any clumps with your fingers.
Preheat the oven to 350°F (175°C) with a rack in the center position. Spray a 9x13-inch baking dish with nonstick cooking spray and set it on a rimmed baking sheet.
Shred the cheddar block on the large holes of a box grater to make about 3 cups, setting aside 1 cup for the topping. Finely chop the onion and melt the butter, dividing it into ¼ cup for the base and ¼ cup for the topping.
In a large mixing bowl, stir together the cream of chicken soup, sour cream, ¼ cup melted butter, onion, garlic powder, salt, and pepper until smooth. Add 2 cups of the shredded cheddar and stir until evenly distributed.
Add the thawed, blotted hash browns and fold gently for about 1 minute until every shred is coated. Transfer to the prepared baking dish and spread into an even layer, pressing lightly into the corners.
Spray the shiny side of a sheet of foil with cooking spray and cover the dish tightly. Bake for 45 minutes, and meanwhile toss the crushed corn flakes with the remaining ¼ cup melted butter.
Remove the foil and sprinkle the reserved 1 cup cheddar and the buttered corn flakes evenly over the top. Bake uncovered for 10 more minutes, until golden and bubbling, and confirm the center reads 165°F (74°C).
Let the casserole rest for 10 minutes so the sauce sets, sprinkle with chives if using, and cut into 12 squares using a 3 by 4 grid.