- 14 tablespoons unsalted butter, softened, and more for the pan
- 6 pounds Yukon Gold potatoes, peeled and cut into chunks
- 2 tablespoons plus 1 teaspoon kosher salt
- 1 ½ cups sour cream
- 1 teaspoon black pepper
- 6 tablespoons finely chopped chives
- ⅔ cup bread crumbs
- ⅔ cup grated Parmigiano-Reggiano cheese
- Lightly grease a 9-inch-by-13-inch baking pan.
- In a large pot, bring the potatoes, 4 quarts water and 2 tablespoons salt to a boil. Boil potatoes until fork tender, about 20 minutes. Drain.
- Mash potatoes with 10 tablespoons butter, sour cream, 1 teaspoon salt, and pepper. Mash in the chives. Taste and adjust seasoning, if necessary. Spread potatoes into the prepared pan. Cover and refrigerate for up to three days.
- In a small bowl, combine the remaining 4 tablespoons butter, bread crumbs and cheese. Mix together until it forms coarse crumbs. Crumbs can be refrigerated for three days.
- Heat the oven to 400 degrees. Sprinkle crumbs over the top of the potato casserole and bake until golden and crisp, 30 to 40 minutes.