Set your sous vide to 194°F.
Rinse and peel the potatoes. Thinly slice them into 1/8 inch pieces.
Smash and peel the garlic cloves.
Seal the potatoes, garlic, rosemary, butter, milk, and salt into a Ziploc or vacuum seal bag (I set my vacuum sealer to 25 seconds) and cook until the potatoes are tender (about 1.5 hours).
Open the bag and pour the liquid through a sieve into a small bowl (reserve for later). Discard the rosemary.
Empty the potatoes into a large bowl and mash them. If you have a potato ricer or food mill, now’s a great time to use it. If you want a really over-the-top, super smooth texture, pass the mashed potatoes through a tamis, chinois, or sieve, using a plastic bench scraper. Whatever you do, DO NOT use a food processor, electric mixer, or blender – due to the starch in the potatoes, if you over mix the potatoes you’ll end up with a gluey texture.
Gently whisk the melted butter and milk (reserved from before) back into the mashed potatoes to get a smooth and creamy texture.