1. Add the potatoes and salt to the Instant Pot insert (6-quart). Add cold water to cover the potatoes by an inch or so. Don’t exceed the max fill line.
2. Secure the lid on the Instant Pot, making sure the valve is set to “seal” and not “vent.”
3. Select Manual and then dial up or down to 8 minutes. The IP will start on its own.
4. While the potatoes cook, warm the milk, butter and garlic powder in a saucepan or microwave-safe container until the butter is melted and the mixture is warm.
5. When the potatoes are done cooking, quick release the pressure. If liquid bubbles and spurts from the valve, cover loosely with a damp dishcloth while the pressure releases.
6. Drain the potatoes in a colander over the sink, discarding the cooking liquid.
7. Return the potatoes to the insert of the Instant Pot and coarsely mash.
8. Add the 2/3 of the butter/milk mixture along with the sour cream, and mash until the potatoes are the desired texture, as smooth or as chunky as you like, adding more of the remaining milk/butter mixture if you want creamier, less stiff mashed potatoes (I usually add it all).
9. Add salt and pepper to taste.
10. Serve immediately OR return the insert to the Instant Pot, secure the lid, and select Keep Warm. The potatoes can stay on this setting for up to three hours.
11. When ready to serve, give the potatoes a good stir and a drizzle of warm milk, if needed.