• 7 large Idaho potatoes (about 3½ pounds/1.5 kg), scrubbed
• 2 cups (480 ml) heavy cream
• 5 sprigs fresh rosemary
• 2 cups (455 g) unsalted butter, cut into small cubes
• Kosher salt and freshly ground black pepper

Preheat the oven to 425°F (220°C). Using a fork, prick the entire surface of the potatoes, then put them on a baking sheet a few inches apart. Bake until a skewer inserted into the center of a potato meets with no resistance, about 1 hour. Let cool for 10 minutes.

While the potatoes are baking, combine the cream and rosemary in a medium pot. Bring to a boil, remove from the heat, cover, and let sit until the potatoes are done, about 1 hour.

Reheat the cream over low heat until just warm, about 5 minutes. Remove the rosemary stems and discard. Halve the potatoes lengthwise and scoop out the flesh; discard the peels. Pass the potatoes through a ricer (in batches) or a food mill set over a large bowl. Add the warm cream, and mix with a whisk until the cream is incorporated. Begin adding the butter a handful of pieces at a time, whisking until fully incorporated. Season with salt and pepper. Serve immediately, or place the bowl over a large pot filled with a few inches of simmering water to keep warm until ready to serve.