1. Preheat oven to 350 degrees. Butter a large shallow baking dish; set aside.

2. Fill a large pot with 2 to 3 inches of water; set a steamer insert into pot and add potatoes to insert. Cover and steam potatoes until tender and easily pierced with the tip of a sharp knife, about 15 minutes. Pass potatoes through a ricer into a large bowl.

3. In a small saucepan, heat butter and milk over medium heat until butter is melted. Add milk mixture to potatoes, along with sour cream, cream cheese, 1 tablespoon salt, and pepper; fold to combine. Transfer potato mixture to prepared dish.

4. In a medium bowl, whisk heavy cream until medium peaks form. Fold in cheese and season with salt and pepper; spoon cream over potatoes, spreading evenly to cover. Transfer to oven and bake until potatoes are heated through and top is light brown, about 30 minutes. Let stand 10 minutes; garnish with chives and serve.