Wash, peel, and chop the potatoes.
Add the rack to the pressure cooker. Place the potatoes on top of the rack. Shake salt over the potatoes. Add a dollop of butter on top. Pour the water over the potatoes.
Using the manual setting, set the cook time for 8 minutes. Do a quick pressure release.
Remove the pot from the pressure cooker and drain the potatoes.
Put the potatoes in the bowl of a stand mixer or another large bowl. Add more butter and salt. Mix, using a stand mixer or hand mixer, adding dribbles of milk as you go. It doesn't take much milk, so go easy.
When you reach the desired consistency, taste and add more salt if needed.