Preheat oven to 400°, then put the potatoes in it to bake.

In the meantime, heat the milk until it's just short of scalding. Add the peppercorns and herbs to the milk, and let them steep at least twenty minutes.

Strain out the peppercorns and herbs, and return the milk to the saucepan. Pureé the cream and garlic cloves together in a blender or spice grinder, and add to the milk.

Slice the butter into a heat-proof bowl large enough to hold all of the potatoes. Toast the coriander seeds in a small skillet until fragrant, grind them in a spice grinder, and sprinkle on top of the butter along with the salt.

When the potatoes are tender, let cool just a bit. Hold each with a dishtowel while peeling it, then transfer the peeled potato to a potato ricer. Squeeze the potatoes directly into the bowl on top of the butter. Stir in the warm cream and milk mixture. Check for consistency and seasonings, and adjust as needed.

If you're not quite ready to serve, place the bowl of potatoes over a pan of simmering water. Cover it up, and continue simmering for up to an hour, stirring occasionally.