Preheat your oven to 400°F. Spread the frozen hashbrowns in a single layer in a 9x13 baking dish. Bake them for 10–12 minutes before adding any other ingredients. This evaporates excess moisture and prevents a "mushy" base.
In a large bowl, whisk together the sour cream, cream of chicken soup, milk, salt, and pepper. Stir in the shredded chicken, cheddar cheese, peas, carrots, and the well-drained corn.
Spread the creamy chicken mixture evenly over the par-baked hashbrowns. Use a spatula to press the mixture down slightly so it integrates with the top layer of potatoes.
In a small bowl, toss the panko breadcrumbs with the melted butter, garlic powder, and smoked paprika. This ensures every crumb is seasoned and will toast to a deep golden brown.
Sprinkle the breadcrumb mixture over the top of the casserole.
Bake for 45 minutes. If the crumbs are browning too quickly, loosely tent the dish with foil for the last 15 minutes.
Let the casserole sit for 10 minutes after removing it from the oven. This "sets" the cream sauce so it stays on the chicken rather than running to the bottom of the plate.