Place potatoes in a large pot and pour in cold water to cover by 1". Add kosher salt and bring to a boil. Reduce heat and simmer slowly until potatoes are very tender but not saturated or crumbly, 20–25 minutes. (Boiling will lead to waterlogged pieces.)
Drain potatoes, reserving 1/2 cup cooking liquid if making potatoes ahead. Return potatoes to pot and set over low heat. Gently stir until dry, about 1 minute.
Pass hot potatoes through a ricer into a large bowl (if allowed to cool, the potatoes will become gummy).
Meanwhile, gently heat whole milk, (thyme and bay leaves, if desired), and Truffle Butter in a small saucepan over medium heat. Remove from heat.
Remove herbs from warm milk mixture; discard. Gradually add milk mixture to potatoes, stirring vigorously with a wooden spoon until combined and smooth; season with salt and pepper.
Serve mashed potatoes with a dollop of truffle butter on top.
If you want to cook the potatoes ahead, make sure to reserve 1/2 cup of the potato cooking water before draining. Rice potatoes and return to the pot; stir in dairy, season, and cover. As serving time approaches, place the pot over low heat and add 1/4 cup of the cooking water, stirring, until the mixture is hot. Add the remaining 1/4 cup cooking water if needed.