Preheat your oven to 350°F (175°C).
Boil the peeled and cubed potatoes in salted water until tender, about 15 minutes.
Drain the potatoes and return them to the pot.
Add butter, cream, milk, garlic powder, salt, and pepper to the potatoes.
Mash the mixture until smooth and creamy.
Stir in sour cream, green onions, and half of the cheddar cheese.
Transfer the mashed potatoes to a greased baking dish.
Top with remaining cheddar cheese and breadcrumbs if using.
Bake in the preheated oven for 20-25 minutes until golden and bubbly.
Let it cool slightly before serving.