Preheat the oven to 350°F (175°C).
In a large pot, bring salted water to a boil.
Add the cubed potatoes and cook until fork-tender, about 15-20 minutes.
Drain the potatoes and return them to the pot.
Using a potato masher or electric mixer, mash the potatoes until smooth.
Add the softened cream cheese and fat-free sour cream to the mashed potatoes.
Mix well until the cream cheese is fully incorporated and the mixture is creamy.
Season the mashed potatoes with salt and pepper to taste.
Transfer the mashed potato mixture to a 2-quart baking dish, spreading it out evenly.
Dot the top of the mashed potato mixture with butter or margarine, then sprinkle with parsley flakes for a pop of color.
Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes.
After 30 minutes, remove the foil and continue baking for an additional 10-15 minutes, or until the top is lightly golden.
Serve the creamy mashed potato casserole hot as a delightful side dish to complement your main course.