Grease a 13 x 9 baking dish with butter. Set aside.
Place potatoes in a 6 quart Dutch oven or large pot and cover with cold water. Bring to a gentle boil and cook (uncovered) until fork-tender (but not falling apart), about 13 to 16 minutes.
Drain potatoes in a colander and return to pot. Mash potatoes (using a potato masher) with cubed cream cheese until combined.
Add sour cream, onion powder, garlic powder, salt and pepper. Mash until combined. Season with additional salt, pepper, garlic powder and/or onion powder, if desired and mash (or stir) again to combine.
Transfer to prepared baking dish and spread evenly.
Allow potatoes to cool to room temperature. Cover tightly with foil and refrigerate overnight (or up to 2 days).
When ready to bake, remove from refrigerator 30 minutes prior to baking.
Preheat your oven to 350 degrees F. Bake (covered with foil) for 50 minutes.
Carefully remove foil and bake an additional 5 to 10 minutes, or until heated through.
Top as evenly as possible with melted butter and chopped fresh chives (or parsley) when serving.