Peel the potatoes and cut into quarters. Place potatoes into the Instant Pot and cover with water. Add salt (I usually add about 1 teaspoon at this point).
Place the lid on the Instant Pot and set the valve to sealing. Cook on Manual at high pressure for 10 minutes. When the timer goes off, turn the Instant Pot off and quick release the pressure.
Drain the potatoes and return them to the Instant Pot. Mash the potatoes with a potato masher or with a hand mixer.
In a small bowl, combine the milk and the butter. Microwave for about 40 seconds. This helps the milk not to curdle or to cool the potatoes down too much. Slowly add in the butter/milk mixture until potatoes reach desired consistency.
Mash in the sour cream, salt, and pepper. Serve immediately.
If you can't serve right away, turn the Instant Pot back on to the Keep Warm setting.