Bring salted water to a boil in a large pot and cook cubed potatoes until tender, about 15 to 20 minutes. Drain and let cool slightly.
Mash the cooked potatoes in a large mixing bowl until smooth.
Stir in sour cream, cream cheese, milk, garlic powder, onion powder, salt, pepper, cheddar cheese, chopped bacon, and sliced green onions until well combined.
Preheat oven to 350°F (175°C). Grease a 9x13-inch baking dish and spread the potato mixture evenly in it.
Sprinkle additional shredded cheddar cheese on top of the casserole.
Bake the casserole for 25 to 30 minutes, or until the cheese is melted and bubbly.
Remove from the oven, let cool a few minutes, then garnish with extra green onions and parsley if desired. Serve warm.