Preheat your oven to 375°F (190°C).
Peel and chop the potatoes into even-sized chunks.
Boil the potatoes in salted water until tender, about 15-20 minutes.
Drain and return the potatoes to the pot; add butter, milk, garlic powder, salt, and pepper.
Mash the potatoes until smooth and creamy, then fold in half of the cheese.
Spread the mashed potatoes in a greased 9x13 inch baking dish.
Layer the shredded chicken on top of the mashed potatoes evenly.
Sprinkle the remaining cheese and breadcrumbs over the chicken.
Bake uncovered for 25-30 minutes or until the top is golden brown.
Let the casserole rest for 10 minutes before serving.