Add the potato chunks to the Instant Pot and cover with water by about 1 inch. Secure the lid and set the valve to sealing. Cook on high pressure for 18 minutes.
Once cooking time is complete, allow a natural release for 10 minutes, then perform a quick release by moving the valve to venting.
Drain the potatoes thoroughly and return them to the pot. Add the warm milk, butter, salt, and pepper. Mash using the 'Mash' function or a potato masher until smooth and creamy.
Transfer to a serving bowl and garnish with chopped chives if desired. Serve immediately.
Notes
For extra creaminess, add an additional tablespoon of butter or a splash of heavy cream when mashing.