Wash and peel the potatoes. Cut each potato into 6 slices.
In an instant pot (I used a 6-quart), stir together the potatoes, water, salt, and pepper. Secure the lid and turn the valve to seal. Set to cook on manual high pressure for 10 minutes. It will take about 10 minutes to build pressure before the timer begins.
When the timer goes off, do a quick release by turning the valve. Let potatoes sit a couple minutes to let any extra steam/liquid evaporate. You shouldn't need to drain much water, but drain any that remains.
Add the butter and sour cream. Mash with a potato masher, adding 1 Tablespoon heavy cream at a time, until you reach your desired consistency. Be careful not to over mash. Salt and pepper, to taste.
Keep warm until ready to serve by placing the lid on loosely and turning on the warm setting.
Notes
Variations:
Use softened cream cheese instead of sour cream
Add garlic cloves to the potatoes before cooking for garlic mashed potatoes
Stir in parmesan cheese for a cheesier potato
Add fresh herbs to the potatoes before cooking, such as rosemary or thyme
Garnish with parsley
Make-ahead:
Once cooked and mashed, place the potatoes in a serving dish.