Preheat the oven to 400 degrees F. Prick the potatoes all over with a fork and roast them in the oven until tender, about 1 hour. Being careful of the hot potatoes, split them and scoop out the flesh into a bowl. Add the butter and season with salt and a few twists of black pepper. Mash until the butter is melted and the potatoes are smooth. Keep warm until ready to serve.