Preheat your oven to 350°F (175°C).
Boil the cubed potatoes in salted water until tender, about 15-20 minutes.
Drain the potatoes and return them to the pot.
Add butter, milk, garlic powder, salt, and pepper to the potatoes.
Mash the potatoes until smooth and creamy.
Stir in the sour cream and half of the cheese until well combined.
Transfer the mashed potatoes into a greased baking dish.
Spread the remaining cheese on top of the potatoes.
Sprinkle breadcrumbs over the cheese layer if desired.
Bake in the preheated oven for 25-30 minutes or until golden and bubbly.