Wash and peel potatoes and cut into quarters. Transfer potatoes to the bowl of an Instant Pot® (or other pressure cooker) and add just enough water to cover. Add the 2 teaspoons salt and 2 tablespoons of the butter.
Set the Instant Pot pressure setting at high for 7 minutes. When done, use the quick release valve to immediately release pressure. Drain potatoes in a strainer and transfer to a heat-proof bowl.
Mash potatoes with a potato masher or ricer. Add mayonnaise, heavy cream and remaining butter and mix well. Salt to taste.
Serve garnished with chopped parsley, freshly ground black pepper and a pat of butter, if desired.