Vanilla-Rosemary Double-Potato Dauphinoise

Preheat oven to 325 degrees F.

Combine milk and cream cheese in a medium saucepan. If using vanilla bean, split the pod in half lengthwise. Use the tip of a paring knife to scrape the seeds into the milk mixture and add the pod to the pot (or add vanilla extract). Cook over medium heat, whisking, until the cream cheese melts and the mixture is steaming. Let stand for 10 minutes. Remove the vanilla bean (if using) and stir in rosemary.

Very thinly slice potatoes and sweet potatoes with a mandoline or by hand. Layer one-third of the sweet potatoes, slightly overlapping, in an 8-inch-square baking dish. Sprinkle with a little salt and pepper. Layer on one-third of the potatoes and sprinkle with a little more salt and pepper. Set aside 1/4 cup of the milk mixture, then pour about half of the remaining mixture over the potatoes. Repeat the same layers two more times. (The potatoes will not be completely submerged.) Drizzle the reserved 1/4 cup milk mixture over the top and sprinkle with cheese.

Bake until bubbly and golden brown, 1 1/4 to 1 1/2 hours. Let cool for about 15 minutes before serving. Cut into 9 squares.