Place potatoes in a big pot and pour cold water over them until they're covered by 1”. Add salt and bring the pot to a boil. Turn the heat down and simmer for 20-25 minutes. The potatoes should be very tender but not saturated or crumbly (boiling will lead to waterlogged pieces).

Drain the spuds, setting aside ½ cup cooking liquid if you're making the potatoes ahead. Return the potatoes to their pot and set over low heat. Gently stir until dry, about 1 minute.

Meanwhile, heat milk, thyme sprigs, bay leaves, and ¾ cup butter in a small saucepan on medium heat until the butter is melted. Take it off the heat.

Pass the hot potatoes through a ricer into a large bowl (it's important to do this while they're hot; if allowed to cool, the potatoes will become gummy).

Remove the herbs from the warm milk mixture; discard. Gradually add the milk mixture to the potatoes, stirring vigorously with a wooden spoon until combined and smooth, then season with salt and pepper.

Serve the mashed potatoes with a few pats of butter on top.