Wash and peel the potatoes. Cut each potato into 6 slices.
Place potatoes, water, salt and pepper in the instant pot (I used a 6 quart). Stir and then place the lid on with the valve turned to seal. Then cook on manual high pressure for 10 minutes. It will take about 10 minutes to build pressure before the timer begins.
Once the timer goes off, do a quick release by turning the valve. Once the lid is off, allow it to sit for a couple minutes to let the extra steam/liquid evaporate. You shouldn't need to drain. Then add in the melted butter and sour cream. Mash with a potato masher right after they are done cooking. Add 1 Tablespoon of heavy cream at a time, until you reach your desired consistency. Be careful not to over mash. Salt and pepper to taste.
Keep the lid on loosely and turn on the warm setting until ready to serve.
Once cooked and mashed, place the potatoes in a serving dish.