Take the whole, unpeeled potatoes to a preheated oven at 200C for 1-2 hours or until really soft and well cooked.
As soon as they are cool enough to handle pass the inside of the potatoes through a potato press or sieve.
Place the mashed potatoes in a pot on medium-low heat and add the butter and milk and adjust with more milk if necessary until you get the consistency you like. Season with salt and pepper.