How to Make It
Preheat oven to 400°F. Spread 2 cups salt on a rimmed baking sheet so it mounds slightly. Arrange potatoes evenly on salt. Bake until tender, 50 to 60 minutes. Remove from oven; let stand until cool enough to handle, about 30 minutes.
Peel potatoes; transfer to a ricer and press into a large bowl, or use a potato masher. (To make ahead, let potatoes stand, covered, at room temperature for up to 6 hours.)
Stir together cream, milk, butter, rosemary, bay leaf, pepper, and remaining 4 teaspoons salt in a medium saucepan. Bring to a bare simmer over medium-low; cook, undisturbed, until butter melts. Remove from heat; let stand, stirring occasionally, for 10 minutes. Remove and discard rosemary and bay leaf.
Reheat cream mixture over medium, stirring occasionally, until warmed through, 5 to 6 minutes. Pour over potatoes and stir gently until smooth and combined. Serve garnished with pepper.
Bake buttery Yukon Golds in salt to intensify flavor, then pass them through a ricer for a smooth texture.