Heat the oven to 250 degrees F. Brush a 9x13-inch baking dish with some of the melted butter. Put the potatoes in a large saucepan and cover with cold water by at least 1 inch. Add 2 teaspoons salt, cover, and bring to a boil over high heat. Lower the heat to maintain a gentle boil, cover the pan partially, and cook until the potatoes are quite tender, 10 to 15 minutes. Drain and return the potatoes to the pan. Over low heat, shake the pan until the potatoes are dry and no longer steam profusely, about 2 minutes.

Pass the potatoes through a ricer (or force through a coarse sieve with a rubber spatula) into a large bowl. Stir in the milk and half of the melted butter with a wooden spoon. Whisk the whipped cream a few times to fluff it up and then gently stir about one-third of it into the potato mixture to lighten it with the whisk. Using a rubber spatula, fold in the rest of the whipped cream. Season with salt and pepper to taste.

Transfer the potatoes to the baking dish. Spread evenly, and drizzle with the remaining butter. Bake the mousseline for 30 minutes until heated through. Just before serving, heat the broiler to high and broil the potatoes until lightly browned, 2 to 4 minutes.

Make Ahead Tips: The potatoes can be mashed, mixed with the milk and cream, spread in the baking dish, and drizzled with butter up to 8 hours ahead. Cover with plastic wrap and refrigerate. Let the potatoes come to room temperature for 30 minutes before baking as directed above.