Roast the potatoes, skin on, in a preheated oven at 200°C/400°F for 1 hour - 1 hour 30 minutes depending on the size of the potatoes. You should be able to puncture them with a knife with extreme ease.
While they're still hot, cut them in half and remove the inside with a spoon. It's important that they're hot or it will be too hard to press them. Discard the skin. Press the potatoes through a potato ricer or through a colander/sieve, pressing with the back of a spoon.
Add the butter, cream, salt and pepper and mix. Adjust the texture to your liking by adding milk.