Sweet potatoes are great to use if you need to make mashed potatoes ahead of time. Reheating them in the microwave does not affect their flavour at all. Go easy on the cinnamon, there should just be a hint of it.
Preheat the oven to 200 C / Gas 6. Grease a large baking tin.
Wash the sweet potatoes and pierce them with a fork several times all over. Place on the baking tin and bake till soft, about 1 hour.
Remove from the oven and set to the side till cool enough to handle. Cut each potato in half and scoop out the flesh, or peel them. Add the flesh to a bowl.
Heat the milk with the butter in a small saucepan till the butter is melted. Add to the potatoes and mix well, then puree till smooth with an immersion blender or in the food processor. Season with salt and cinnamon and serve right away, or refrigerate and reheat throughly in the microwave.