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. You can tell when potatoes are done, if when pierced with a fork they break in half. Drain, reserving a cup of the starchy water that potatoes cooked in.
Mash potatoes with 10 tablespoons butter, sour cream, 1/2-3/4 cup starchy potato water (just enough to get a creamy, not dry texture), 1 teaspoon salt, and pepper. Stir in the chives. Taste and adjust seasoning, if necessary. Spread potatoes into the prepared pan. Cover with plastic wrap and refrigerate for up to three days. Even if your casserole dish has a lid, use plastic wrap. It will better seal in the moisture and keep the potatoes from drying out.
In a small bowl, melt the remaining 4 Tablespoons butter in the microwave in 15 second increments. combine 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, uncovered until golden and crisp, 30 to 40 minutes.
Garnish with additional chives, or fresh thyme works nicely here. Serve directly from the casserole pan, or spoon into your favorite special serving dish.