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

In a small saucepan, combine the heavy cream and unsalted butter over medium heat until the butter melts. Add minced garlic and fresh rosemary and steep for about 10 minutes, then remove rosemary and discard garlic.

Mash the drained potatoes until smooth, gradually mixing in the warm cream mixture. Season with additional kosher salt and black pepper to taste.

Transfer to a serving dish and garnish with a pat of butter and fresh rosemary leaves. Serve warm.