Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish with melted butter or cooking spray.

Peel and dice the Yukon Gold potatoes. Bring a large pot of salted water to a boil, then add the potatoes. Cook for about 10-15 minutes until fork-tender, then drain and cool slightly.

In a large mixing bowl, combine sour cream, cream of chicken soup, shredded cheddar cheese, diced onions, melted butter, garlic powder, salt, and pepper. Mix until blended.

Fold the drained potatoes into the creamy mixture, ensuring they are evenly coated but not mashed.

Spread the potato mixture into the prepared baking dish and smooth the top with a spatula.

Generously sprinkle crushed cornflakes or potato chips over the top of the casserole.

Bake for 30-35 minutes until bubbly around the edges and golden brown on top.

Remove from the oven and let cool for about 10 minutes before serving.