Place peeled potatoes in a large pot of water and bring to a boil. Boil until fork tender, 20 to 30 minutes.
Preheat the oven to 350 degrees F (175 degrees C). Spray a 3-quart casserole dish with butter spray.
Mash cooked potatoes using a potato masher in a bowl. Add 1/4 cup milk, salt, and pepper to taste. Depending on consistency, add more butter or milk. Stir in sour cream and 1/2 cup Cheddar cheese; mix well. Transfer mixture to prepared casserole dish.
Bake in the preheated oven until heated through, about 20 minutes. Top with remaining Cheddar cheese, crumbled bacon, and chives. Bake until cheese has melted, about 5 minutes.
This dish can be made one day in advance. Cover the casserole dish prior to baking and store overnight in the refrigerator. Take out 30 minutes prior to baking and bake (the cooking time may need to be increased). Soften potatoes up by adding a little more milk and butter prior to baking.
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