4 1/2 to 5 pounds Idaho russet potatoes, peeled and cut into 1-inch chunks (see Cook's Note)
3/4 cup whole milk
1 stick butter, cut into slices, plus 1 additional tablespoon butter for baking dish
1 tablespoon unseasoned breadcrumbs
1 cup heavy cream
3/4 cup grated Parmesan cheese
Make sure the potato chunks are all relatively the same size to assure even cooking time.