Add water to a 6 quart Instant Pot, followed by steam rack. Stack potatoes on top of steam rack.
Close and lock lid in place making sure Pressure Release Valve is set to sealing. Set Instant Pot to Manual Pressure High for 20 minutes.
Instant Pot will take about 15 minutes to come to pressure then countdown from 20 minutes. Once 20 minutes is complete, do not release the Pressure Valve. Let Instant Pot Natural Pressure release. This will take about 15 minutes. Once pressure pin has dropped, remove Instant Pot lid.
Using kitchen tongs, carefully remove potatoes from Instant Pot and transfer to a large mixing bowl for mashing. Using a potato masher, add salt and butter to potatoes and begin mashing. Once potatoes are mashed, stir in warm half and half. Sprinkle with freshly ground black pepper and adjust seasoning to taste. Sprinkle with fresh parsley or fresh chives before serving.
To keep Instant Pot Mashed Potatoes warm, remove steamer rack from Instant Pot, rinse stainless steel Instant Pot liner with hot water to remove leftover potato starch and dry with lint free dish towel. Return liner to Instant Pot and set to Keep Warm. Add one tablespoon of butter and let melt to coat inside of Instant Pot liner. Add mashed potatoes back into Instant Pot, top with additional butter and keep warm, stirring occasionally.
If you have a nylon potato masher, you can skip transferring the potatoes to another bowl for mashing and just mash them right in the Instant Pot.