Preheat the oven to 350°F (175°C). Grease a 9×13 inch casserole dish with butter or non-stick spray.
Place the diced potatoes in a large pot and cover with cold water. Bring to a boil over high heat and cook until tender, about 10-12 minutes. Drain well.
In a large mixing bowl, combine the cooked potatoes, shredded chicken, 1 cup of shredded cheddar cheese, sour cream, mayonnaise, chopped onion, parsley, garlic powder, onion powder, salt, black pepper, and milk. Mix gently until well combined.
Transfer the mixture into the prepared casserole dish and spread evenly.
In a small bowl, mix the melted butter with the crushed cornflakes. Sprinkle this mixture evenly over the top of the casserole.
Sprinkle the remaining 1/2 cup shredded cheddar cheese over the cornflake topping.
Bake in the preheated oven for 35-40 minutes, or until the casserole is bubbly and the topping is golden brown.
Remove from the oven and let it rest for 5 minutes before serving.