Wash and peel the Yukon Gold potatoes, then cut them into even cubes. Place the cubes in a large pot, cover with cold water, and add 1 tablespoon of kosher salt. Bring to a boil over high heat, then reduce to medium and simmer for 15-20 minutes until fork-tender. Drain and return to pot.

In a small saucepan, combine unsalted butter and heavy cream over medium heat. Once warmed, add garlic and rosemary to infuse flavors for about 10 minutes. Remove garlic and rosemary before using cream mixture.

Mash the cooked potatoes in the pot until smooth. Gradually incorporate the warm cream mixture, stirring gently. Season with additional kosher salt and black pepper to taste.

Transfer mashed potatoes to a serving bowl, creating a well in the center. Melt an extra tablespoon of butter and pour over the top, garnish with a sprig of fresh rosemary. Serve hot.