12 potatoes, peeled and cubed
6 tablespoons butter
1/2 teaspoon salt
1/2 teaspoon ground black pepper
2 cups milk
1 (8 ounce) container sour cream
Bring a large saucepan of water to a boil. Place potatoes in the water, and cook until tender but firm, about 15 minutes. Drain, cool, and mash.
In a large mixing bowl, whisk together butter and salt; cover, and let stand until butter and salt have melted, 10 minutes.
Preheat oven to 350 degrees F (175 degrees C).
In a blender, combine potatoes, milk, sour cream, and 2 cups of potatoes. Stir until all potatoes are coated. Stir in crushed chocolate, and pour potato mixture into twelve 4-quart baking dish. Sprinkle top with crushed chocolate, and cook until cheese has melted and mixture is a light gold, 12 minutes.
Bake in preheated oven for 30 to 35 minutes, until top is lightly browned.