Fill a large pot with water and place a sous vide immersion cooker into the water. Set temperature to 195 degrees F (91 degrees C) according to manufacturer’s instructions.
Peel potatoes and cut into cubes, no bigger than 3/4 inch. Place potatoes in a single layer inside a large vacuum bag, along with cubed butter, 1/2 cup half-and-half, and sour cream. Ensure potatoes are spread out as much as possible. Seal the bag using a vacuum sealer, stopping the sealer early so liquid doesn’t escape from the bag.
Place bag into the water and set timer for 90 minutes. Some air may remain in the bag, and you will need to weigh the bag down with a heavy object to ensure bag is fully submerged. After 45 minutes, remove bag and redistribute potatoes evenly in bag, if needed.
When timer is up, cut bag open and dump contents into a large bowl. Season with salt and pepper and mash potatoes. Add in additional half-and-half, if needed.
Feel free to add different seasonings to the bag to suit your tastes. I like to use truffle or garlic butter instead of plain.
To make vacuum sealing liquids easier, you can freeze half-and-half and sour cream in ice cube trays ahead of time.
If you do not have a vacuum sealer, you may use a large plastic resealable bag. To use the water displacement method to seal a plastic resealable bag, slowly lower the bag into the water, letting the water pressure force the air out of the bag, then seal closed. Halfway through cooking, move potatoes around in the bag so they cook evenly.
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