Wash and peel (if desired) the Yukon Gold potatoes. Cut them into evenly sized cubes. Place in a pot, cover with cold water, and add kosher salt. Bring to a boil, then simmer for 15-20 minutes until fork-tender. Drain and keep warm.

In a saucepan, combine heavy cream and unsalted butter. Heat over medium until melted. Add garlic and fresh rosemary; steep for 10 minutes. Remove rosemary and mash garlic into cream for stronger flavor.

Mash the drained potatoes. Gradually pour in the cream mixture, stirring to mix evenly. Season with salt and black pepper to taste. Avoid overworking the potatoes.

Transfer mashed potatoes to a serving bowl. Garnish with fresh rosemary leaves and melted butter. Serve hot.