Peel the potatoes and quarter them (halve any smaller potatoes - you want all the pieces to be about the same size for even cooking). Place the cut potatoes in the Instant Pot.
Add enough water to just cover the potatoes. In my 6-quart Instant Pot, 4 cups of water does the trick. Add the salt and give everything a quick stir.
Put the lid on the Instant Pot and turn the steam release valve to SEALING. Cook at HIGH pressure for 10 minutes. Note: It will take about 15 minutes for the Instant Pot to come up to pressure before the cooking time begins to count down.
While the potatoes are cooking, gently warm the milk; set aside. You can do this on the stove top or in the microwave.
When the cooking time has finished, immediately and carefully quick-release the pressure.
Drain the potatoes in a colander and return them to the hot pot. Allow them to sit for 2 to 3 minutes, stirring once or twice, or until the excess water has evaporated out and the potatoes are dry. Dry potatoes = fluffy mashed potatoes. You can put the pot back in the Instant Pot on Keep Warm with the lid off for a few minutes to help speed up the drying time.
Add the butter and mash the potatoes until most of the lumps are gone. Be careful not to over-mash/over-mix your potatoes so they don't become pasty and gummy.
Add the warm milk; continue mashing and stirring until the potatoes have reached the consistency you like, adding more milk if desired. Taste and add more salt if needed.