1) Preheat the oven to 180°C / gas mark 4
2) Cook the bacon under the grill, 12 to 15 minutes until crisp. Cool and chop.
3) Put potatoes in a medium saucepan and cover with cold water. Bring the water to a boil and season with salt. Cook until tender, about 15 minutes.
4) Drain and mash potatoes with buttermilk and season with salt and pepper.
5) Stir in the chives, cheddar and fold in bacon. If you want to make this ahead of time, at this point you can cool and transfer to a freezer-safe container. Remove from the freezer and allow to defrost at room temperature, then simply reheat in the microwave, stirring often until piping hot all the way through.