Wash the Yukon Gold potatoes thoroughly, peel if desired, and cut into 1-inch cubes. Place in a large pot and cover with cold water. Add 1 tablespoon of kosher salt, bring to a boil over medium-high heat, reduce heat, and simmer for 15-20 minutes until fork-tender. Drain well.

In a saucepan, combine heavy cream and unsalted butter over medium heat, stirring gently until melted. Add garlic cloves and rosemary, steep for 10 minutes, then remove rosemary. Optionally mash garlic into the cream.

Return drained potatoes to pot over low heat. Mash until smooth and fluffy, gradually mixing in warm cream mixture. Season with salt and black pepper to taste.

Transfer mashed potatoes to a serving bowl, garnish with fresh rosemary and a pat of butter, and serve hot.