Place the potatoes in a large saucepan. Cover with water and add salt. Bring to a boil. Cook over medium-high heat until tender, about 45-50 minutes. Drain and peel the potatoes.
Mash the potatoes in a large bowl with potato masher or back of fork. Add the butter, cream cheese, milk, sour cream, 1/2 cup of cheddar cheese, chilli powder, salt and pepper. Stir well.
Preheat oven to 350 degrees F. Spray a baking dish with cooking spray. Place the potato mixture in the baking dish.
Bake for 20-25 minutes. Remove from oven and sprinkle remaining cheddar cheese on top. Return to oven and bake until cheese melts. Garnish the potato casserole with sundried tomato and green onion. Serve and enjoy!