- In a large pot of salted water, cook the potatoes until tender. Drain.
- With the rack in the middle position, preheat the oven to 200 °C (400 °F). Butter a 2 litres (8 cups)-capacity baking dish.
- With a potato masher, crush the potatoes with the butter. With an electric mixer, purée the potatoes with the milk and sour cream. Stir in 250 ml (1 cup) of the cheese, the bacon and chives. Season with salt and pepper.
- Spread into the baking dish. Sprinkle with the remaining cheese. Bake for about 40 minutes or until the crust is golden brown.