You don't have to peel the potatoes, so start buy cutting them into equal sized cubes.
Place the potatoes into a bag for sous vide. (Read notes below about vacuum sealing). Add the milk, butter, and salt to taste. Remove as much air for the bag as you can.
Place the bag into a water bath. Set your sous vide machine for 185 degrees for 1 hour.
If the bag floats at all use a small pot to keep it down or you could clip the part to the side of whatever you are using for your water bath. Make sure to not go above the max line for your machine.
Carefully remove the bag after an hour from the water bath using a long pair of tongs. Allow the bag 5 minutes to cook before opening.
Set a sieve over top a large measuring cup or bowl. Drain the milk, making sure you save the milk.
Place the potatoes into a mixing bowl. Add a little bit of the milk and using a hand mixer start to whip the potatoes, after a few seconds if the potatoes seem dry, add more milk and continue to whip until you have the right consistency. Try to keep the amount of time whipping the potatoes to 30 seconds and no more than 1 mintue.
Taste and add more salt if necessary.
To keep the potatoes warm, place them back in the bag or a new bag if needed, making sure it's water tight. Seal the potatoes up and place them into a 150 degree water bath until you are ready to eat.