When it beeps, turn to Quick Release. When the valve drops, remove the cover carefully. Drain the water completely but keep the potatoes inside the Instant Pot. Use a potato mash to mash the potatoes to your desired consistency. (I like tiny potato chunks in my mashed potatoes).
Turn on the Saute Mode and add the butter, whole milk, salt and pepper into the potatoes. Stir to combine well until the mashed potatoes become creamy. Turn off Saute mode, garnish the mashed potatoes with chopped parsley and serve immediately