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Place potatoes in a large pot and pour in cold water to cover by 1”. Add salt and bring to a boil. Reduce heat and simmer until potatoes are very tender but not saturated or crumbly, 20–25 minutes (boiling will lead to waterlogged pieces).
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Drain potatoes, reserving ½ cup cooking liquid if making potatoes ahead. Return potatoes to pot and set over low heat. Gently stir until dry, about 1 minute.
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Meanwhile, heat milk, thyme sprigs, if desired, bay leaves, and ¾ cup butter in a small saucepan over medium heat until butter is melted. Remove from heat.
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Pass hot potatoes through a ricer into a large bowl (if allowed to cool, the potatoes will become gummy).
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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.
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Serve mashed potatoes with a few pats of butter on top.
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Do Ahead: Potatoes can be made 4 hours ahead. Pass through ricer into a large pot and cover. Just before serving, reheat over low, gently stirring in reserved potato cooking liquid as needed to loosen.
