Preheat oven to 375°F. Line large rimmed baking sheet with foil.
Wash and dry potatoes. Place in a single layer on baking sheet and bake until soft when lightly squeezed, about 1 hour. Rest on pan until cool enough to handle.
Cut potatoes in half and scoop out potatoes into a large bowl. Whip potatoes with an electric hand mixer or mash with a potato masher. Add milk, salt, pepper and butter to the bowl. Stir until butter is melted and ingredients are blended.
Cover with plastic and refrigerate for up to 2 days. Heat in microwave or large saucepan over medium heat. Top with more butter and chives before serving.
Prepare mashed potatoes according to recipe instructions. Transfer to foil pan or other freezer-safe container. Cover, label and freeze up to 3 months. Thaw in refrigerator before re-heating according to step 4.