Put the potatoes in a 10-quart pot and add the cold water. Bring to a boil over high heat, then reduce the heat and simmer for 15–20 minutes, until the potatoes are very tender when pierced with a fork.

While the potatoes cook, warm the milk in a small saucepan over medium-low heat until steaming but not boiling.

Drain the potatoes well and return them to the pot. Mash by hand with 6 tablespoons of the butter, the cream cheese, sour cream, warm milk, salt, and pepper until creamy.

Coat a 9x13-inch baking dish with nonstick cooking spray and transfer the potatoes to it. Smooth the top and dot with the remaining 2 tablespoons butter.

Let the potatoes cool uncovered for 20–30 minutes.

Cover tightly and refrigerate within 2 hours of cooking for up to 2 days.

When ready to serve, heat the oven to 350°F.

Bake the cold, covered potatoes for 50–60 minutes, uncovering for the last 10 minutes, until the center reaches 165°F.