In a 1 1/2-quart saucepan, combine chicken broth, onion powder, garlic powder, poultry seasoning, parsley, paprika, and salt and pepper to taste. Bring to a simmer.
Whisk together milk and flour until smooth. Pour into saucepan, whisking continuously, until the mixture bubbles and thickens, about 1 minute. Remove from heat.
Adjust an oven rack to the middle position and preheat the oven to 350 degrees. Coat a 9-inch by 13-inch baking dish with nonstick spray.
In a large bowl, add sour cream, hash browns, cheddar cheese, and salt and pepper to taste. Stir to combine, then pour into prepared baking dish.
Bake uncovered for 1 hour, then remove from oven. Melt butter in the microwave. Add corn flakes. Drizzle with melted butter, then spread over the top of the casserole in an even layer.
Bake uncovered 20 minutes longer, then garnish with parsley if desired.
