How to Make It
Preheat oven to 375ºF. Grease a 13- x 9-inch baking dish with softened butter. Set aside.
Place potatoes, 2 tablespoons of the salt, and water to cover in a large saucepan. Bring to a boil over medium-high. Reduce heat to medium, and cook until tender, 15 to 20 minutes. Drain potatoes, and return to pan. Mash potatoes with a potato masher until completely smooth. Place pan over medium-low.
Add half of the butter cubes to potatoes, and cook, stirring constantly, until incorporated, about 2 minutes. Repeat with remaining half of butter cubes. Stir in milk, sour cream, pepper, and remaining 1 teaspoon salt.
Transfer potato mixture to prepared baking dish. Top with cheese and bacon. Bake in preheated oven, uncovered, until cheese is melted, about 5 minutes. Sprinkle with chives, and serve immediately.