Place the potatoes in a large pot and cover with cold water. Add 1 tablespoon of salt and bring to a boil. Reduce heat to medium and simmer for 15-20 minutes, or until the potatoes are fork tender.
While the potatoes are cooking, heat the cream and butter in a small saucepan over medium heat until the butter melts and the mixture is warm.
Add the garlic cloves and rosemary sprigs to the warmed cream mixture. Let it steep for 10 minutes to infuse the flavors. Remove the rosemary sprigs and discard the garlic cloves (or mash them in if you prefer a stronger garlic flavor).
Drain the potatoes well in a colander. Return them to the pot and use a potato masher, ricer, or hand mixer to mash them until smooth and creamy.
Gradually add the warm cream mixture to the mashed potatoes, mixing until well combined. Season with salt and pepper to taste.
Serve hot, garnished with additional fresh rosemary sprigs and butter, if desired.
