Potatoes Mousseline

? cup unsalted butter, melted
2 pounds russet potatoes, peeled and cut into lengthwise quartes
? cup whole milk
1 cup heavy cream, whipped to soft peaks
freshly ground black pepper

Brush 9×13 baking dish with some of the butter.
Put potatoes in sauce pan and cover with cold water by one inch. Add two teaspoons salt, cover, and bring to boil. Cook until potatoes are very tender (15 minutes). Drain and return to pan. Over low heat, shake the pan until the potatoes are dry and no longer steam, about 2 minutes.
Pass potatoes through a ricer into a large bowl. Stir in milk and half of the melted buter with a wooden spoon.
Whisk the whipped cream a few times to fluff it then gently stir about a third of it into the potato mixture with a whisk, to lighten it.
With a rubber spatula, fold in the rest of the whipped cream. Season with salt and pepper to taste.
Transfer potatoes to baking dish. Spread evenly and drizzle with remaining butter.
Broil until lightly browned on top, 2-4 minutes.