Preheat the Sous Vide Machine: Fill a large container or pot with water. Attach the sous vide precision cooker and set the temperature to 194ºF/90ºC and time to 1 hour.
Prepare the Potatoes: Wash and peel the potatoes, and cut them into 1/2-inch slices (you can also leave the skins on for extra flavor).
Place the potato slices into a zip-top bag, then season with butter,salt, and garlic and add milk. Spread out the potatoes evenly in the bag and then seal it using the “water displacement” technique. (Just seal all but one corner of the bag. Slowly place it in the water bath, and make sure everything below the zip-line is covered by water. Then seal the rest of the bag.)
Make sure the potatoes are fully submerged in the water. You can add heavy kitchen items to weigh the bag down if necessary.
Sous vide cook the potatoes: Cook the vacuum-sealed potatoes in the water bath for 1 hour.
Mash the potatoes: When the timer goes off, transfer the cooked potatoes to a large bowl, and add half of the liquid from the bag. Mash the potatoes with a potato masher until smooth, adding more liquid gradually if needed to your desired consistency. (Don’t mix too much or it will get gummy.)
Taste and season: Add in extra salt and garnish with chopped chives and serve warm. You can also sprinkle chopped green onions, freshly grounded black pepper, add more butter, or other herbs/ seasonings!