In a medium saucepan, combine the cream, milk, and the 2 sticks of butter with the rosemary, sage, and garlic and bring just to a simmer. Remove from the heat and let steep for 15 minutes, then discard the rosemary, sage, and garlic. Meanwhile, in a large pot, cover the potatoes with water and bring to a boil. Add a generous pinch of salt and simmer over moderate heat until tender, about 20 minutes. Drain well, then pass the potatoes through a ricer (or mash/whip them thoroughly). Fold in the cream mixture and season generously with salt and pepper. Light the broiler and position the rack 8 inches from the heat. Scrape the potatoes into a 12-inch round flameproof pan or baking dish (2 inches deep) and, using a spoon, decoratively swirl the top. Gently brush with melted butter. Broil for about 8 minutes, until the top is browned in spots. Serve hot and sprinkle with additional fresh herbs, if desired.

This recipe can be prepared in advance and stored in the refrigerator, saving the broiling step until the day you intend to serve it. If you choose this option, reheat the potatoes gently on the stovetop before transferring into the baking dish and broiling.