Prep the potatoes: Place the peeled and cubed potatoes into the bottom of the Instant Pot. Ensure they are cut into even-sized cubes to promote uniform cooking.
Cook the potatoes: Cover the potatoes with the water and stir in the salt. Place the lid on the Instant Pot and set the valve to seal. Cook on manual pressure for 8 minutes. When the timer goes off, turn the Instant Pot off. Quick release the pressure from the pot, allowing the steam to escape carefully.
Finish the potatoes: Drain the potatoes and return them to the Instant Pot. Add butter, buttermilk, and white pepper to the potatoes. Mash until smooth, using a potato masher or a hand mixer for a creamier texture. Garnish with parsley if desired and serve warm.
