Fill a large pot with about 2 inches of water and add a pinch of salt. Set steamer basket in pot, making sure water doesn't seep through holes. Bring to a boil, then reduce to a rapid simmer.
Add potatoes to steamer basket and steam until tender when pierced with the tip of a paring knife, 25 to 30 minutes.
Bring cream, garlic, and rosemary to a simmer in a small pot, then remove from heat and let steep 30 minutes. Strain through a fine sieve. Clean pot, then return cream to pot. Add butter and heat over medium until melted, stirring to combine. Cover and keep warm, either over lowest setting on the stove or in a warm pot.
Remove potatoes from steamer basket and while the potatoes are still hot, pass through a ricer or a food mill fitted with a fine disk into a large bowl. Stir in cream mixture and season with salt and pepper. If a finer texture is desired, pass puree through a medium-mesh sieve, pressing on solids with a rubber spatula to extract as much puree as possible. Reheat over medium heat with a little more cream or water if necessary before serving.