Preheat the oven to 180°C/350°F/gas 4. Wrap the potato loosely in foil and place it on a baking tray. Bake for 75 minutes or until the potato is tender when pricked with a fork. Remove the potato from the oven, remove the foil, and place the potato on a wire rack to cool slightly. Peel the potato and place the flesh in a large bowl. Mash well and stir in the butter, milk and chives. Season to taste and serve.