In a medium saucepan, heat cream, milk, two sticks of butter and garlic together to a slight simmer. Remove from heat and let garlic flavor the liquid. Discard whole pieces of garlic after 15 minutes.
While garlic is steeping, add diced potatoes to a large pot and cover with water. Bring to a boil add 1 tablespoon of kosher salt, then let simmer over medium heat until potatoes are fork tender, 15-20 minutes. Drain potatoes well.
Use a potato masher to mash potatoes until smooth, then stir in butter and cream mixture. Season with additional kosher salt and freshly ground black pepper to taste. Transfer hot potatoes to oven safe baking dish.
Use a spoon to swirl mashed potatoes, then brush with additional melted butter and place under broiler for 5 minutes or until well browned. Watch potatoes closely.
To make ahead, refrigerate potatoes for for up to 2 days after swirling, then heat in microwave until hot. Proceed to brush with melted butter and broil.