Place the potato chunks in the Instant Pot. Add water or stock, salt lightly, and, if using, garlic. Seal the lid and set the valve to sealing.
Cook on high pressure for 8 minutes. Quick-release the pressure when the timer beeps. Steam escapes, and you can hear the potatoes sigh in relief.
Open the lid and mash briefly with a potato masher right in the pot. Add butter and warm milk, then stir until smooth and creamy. If it’s stubborn, give it another minute of gentle mashing.
Season with salt and pepper to taste. If you want ultra-silky potatoes, whisk vigorously for 30 seconds or use a hand mixer on low.
Garnish if you like—think a sprinkle of chives or a little extra butter on top. Serve hot and enjoy the ooey-gooey goodness.