Heat up your Sous-Vide to 90 degrees (c).
Peal the potatoes and cut them into ~1cm thick slices. Pealing is optional.
Cut the garlic into four (half and half).
Cut the butter into pieces (for better distribution)
Insert the potatoes, garlic, butter, salt and rosemary into the vacuum bag.
Vacuum the bag using a proper machine or the submerge technique. I have a vacuum sealer.
Insert the vacuum bag to the pot and wait for 30 minutes.
Take out the potates and put them in a bowl.
Smash & Serve!