Preheat your oven to 400-degrees and place a pie of tinfoil on the top rack. Scrub and pat dry the potatoes, prick each several times with a fork. Place the potatoes on top of the tinfoil and bake under fork tender, about 1 hour.
Remove from the oven to a cutting board. Use an oven mitt to hold the potato while you slice each in half lengthwise. Let the potatoes sit for 10 minutes.
Into a large mixing bowl, add the butter (cut into pieces). Scoop out the potato with a spoon and add it to the bowl. Discard the skins. Using a potato masher, break up the potatoes and incorporate the butter. Stir in the brown sugar and add a little of the milk at a time until you have the desired consistency. Season with salt and pepper.
Transfer the mashed sweet potatoes into a baking dish with a lid. Add an additional pat or two of butter to the top (optional) and cover.
When you want to serve the potatoes, place the dish an oven pre-heated to 350-degrees and bake until heated through.