Place Potato chunks into a bowl of water and let sit for 30 minutes.
Add 1.5 cups of water to Pressure Cooker cooking pot and place a trivet or steamer basket inside. Add potatoes to the trivet/steamer basket.
Lock on lid and close Pressure Valve. Cook on High Pressure for 4 minutes. When Beep is heard, release all pressure.
Place potatoes into a large bowl and mash. Add Butter, Cream Cheese, Heavy Cream, Egg (if using) and all Seasonings. Mash until combined.
Add Mashed Potatoes to Buttered Casserole Dish and top with pieces of Butter. Place Casserole Dish into pressure cooker on top of trivet.
Lock on lid and close Pressure Valve. Cook on High Pressure for 7 minutes. When Beep is heard, release all pressure.
Preheat oven to 350 degrees Fahrenheit. Butter 8 inch cast iron skillet or 4 quart baking dish. Spread the potatoes in the prepared baking dish. Place pieces of Butter over the top of the potatoes. Bake in oven for 25 minutes.
Soaking the Potatoes before cooking, will release much of the starch and make for a fluffier potato.The added egg will allow the potatoes to rise a bit and give a soufflé like texture.Serve with Pot Roast or Beef Stew.