Position an oven rack in the center of the oven and preheat to 350°F (175°C). Brush the inside of a 9x13-inch baking dish with the 1 teaspoon of melted butter you set aside, covering the corners and sides.
Confirm that the hash browns are fully thawed, with no icy clumps. Pour them into a colander set in the sink and let drain for 5 minutes. Press down gently with your hands to squeeze out extra moisture.
In a large mixing bowl, combine the cream of chicken soup and sour cream. Pour in the rest of the 1/2 cup (115 g) melted butter, then add the onion powder, salt, and black pepper. Whisk for about 1 minute until smooth.
Stir the shredded sharp cheddar into the creamy base until evenly distributed. Add the drained hash browns in two or three handfuls and fold with a silicone spatula until every shred is coated, about 1 minute.
Scrape the filling into the prepared baking dish. Spread it into an even layer, pressing gently into the corners, and smooth the surface with the back of the spatula.
Place the corn flakes in a gallon zip-top bag and press out the air. Roll over the bag with a rolling pin until the flakes break into coarse pieces about the size of a dime. Pour into a bowl, add the remaining 1/4 cup (57 g) melted butter, and toss until glossy.
Scatter the buttered corn flakes over the potatoes in an even layer, going all the way to the edges. Slide the dish onto the center rack and bake for 45 minutes. Check at 30 minutes and tent loosely with aluminum foil if the topping is already deep golden.
Look for bubbling sauce around the edges and a golden brown topping. Confirm the center reads at least 165°F (74°C) on an instant-read thermometer. Let stand for 5 minutes before serving.