Bring a large pot of salted water to a boil. Drop in potatoes, and cook until tender but still firm, about 15 minutes.

Transfer potatoes to a large bowl, and mash until smooth. Mix in the cream cheese, sour cream, onion powder, salt, pepper and butter. Cover, and refrigerate 8 hours, or overnight.

Preheat oven to 350 degrees F (175 degrees C). Lightly grease a medium baking dish.

Spread potato mixture into the prepared baking dish, and bake in the preheated oven about 30 minutes.