Mix the potatoes with all the ingredients (except the extra ¼ to ½ cup buttermilk).
Place in a vacuum sealable bag (or a zip lock freezer bag if using the water displacement method) and vacuum seal.
Place the bag in the water bath and cook for 1 hour.
Remove from the water bath and dump potatoes in a bowl.
Add the additional ¼ cup of warm buttermilk (warm it first to keep the potatoes from getting cold) and use a potato masher or ricer to mash the potatoes. (Be careful not to over mash or the potatoes will be gluey. It's ok if there are a few chunks in there).
Add additional buttermilk as necessary to achieve desired consistency.
Add additional salt to taste and serve with turkey gravy and chopped chives.
Notes
Don’t skip the step of removing air from the bag as this keeps bacteria out.
Make sure the bag is completely submerged in the water bath. If the bag floats, weigh it down with a heavy towel or sous vide sinker weight.
If the bag fills with air, you can just open it, release the air and reseal it again.
For even cooking, organize the potatoes in the sealable bag in a single layer and make sure they’re cut into similar sized pieces.
I don’t normally peel the potatoes for this recipe, but if you prefer your mashed potatoes without potato skins, you can peel the potatoes before chopping them into cubes.
Make sure the extra buttermilk is heated before adding it to the mashed potatoes. It doesn’t have to be hot but slightly warmed will ensure it doesn’t bring down the temperature of the mashed potatoes.
You need to be careful not to over mash the potatoes as they might take on a gummy texture.
For the best results, I prefer to use a potato masher or ricer to mash the potatoes instead of a food processor or electric mixer.