Bring a large pot of water to a boil.  Cook the potatoes until tender, about 15-20 minutes.  Transfer potatoes to a large bowl, and rice with a potato ricer, or mash. 

     Mix in cream cheese, sour cream, onion powder, salt, pepper, and butter.  Cover and refrigerate for 8 hours or overnight.  

     Bake at 350 degrees for 30 minutes or place in a slow cooker for about 1-1 ½ hours.