Step 1

Cut 6 tablespoons unsalted butter into 12 pieces and let sit at room temperature to soften. Coat an 11x7-inch or other 2-quart baking dish with unsalted butter. Arrange a rack in the middle of the oven and heat the oven to 400ºF. Meanwhile, prepare the potatoes.

Step 2

Peel 2 1/2 pounds Yukon gold potatoes and cut into 1-inch cubes. Place the potatoes and 2 tablespoons kosher salt in a large saucepan, and add enough cold water to cover the potatoes by about an inch. Bring to a boil over high heat. Reduce the heat and simmer until the potatoes are very tender and can be easily pierced with a sharp knife with no resistance, 10 to 12 minutes. Drain and set aside until the potatoes are cool enough to handle but still warm. Meanwhile, finely grate 2 ounces Parmesan cheese (about 1/2 cup).

Step 3

Pass the warm potatoes through a ricer or food mill into a large bowl. Add the butter and 1/2 cup whole milk, and mix well until combined and the butter is melted.

Step 4

Pour 1 cup cold heavy cream into the bowl of a stand mixer fitted with the whisk attachment. (Alternatively, use an electric hand mixer and large bowl). Beat on medium-high speed to stiff peaks, 3 to 5 minutes.

Step 5

Stir 1/3 of the whipped cream into the potatoes. Gently fold the remaining whipped cream into the potatoes with a rubber spatula. Gently transfer the potatoes into the prepared baking dish. Sprinkle the Parmesan over the potatoes in an even layer.

Step 6

Bake until the potatoes are warmed through and the cheese is lightly browned, 20 to 25 minutes. Let rest 10 minutes before serving.