Preheat the oven to 220 °C. Prick or puncture the sweet potatoes with a fork and put them each onto a piece of foil large enough to wrap them in.
I find these cook best in individual parcels, so just be prepared to cook a lot of them: I find that 3 kg means you are dealing with eight or nine sweet potatoes in one go.
Divide the oil between the foil sheets and then rub or turn the sweet potatoes in it and wrap into baggy but well-sealed parcels.
Put them on a baking sheet or two and roast until the potatoes are soft and cooked through, about one to one and a half hours.
This way of cooking the sweet potatoes, apart from being very easy, means that you get maximum, unwatered-down flavour.
When the sweet potatoes are cool enough to handle, strip away the skin and fork or squeeze and pick the flesh into a large bowl.
Pour in any syrupy juices from each of the foil parcels.
Add all the other ingredients except the marshmallows, and mix together to make the mash.
Spoon the sweet potato mash into an ovenproof dish (I use an old rectangular one of 34 x 20 cm and about 6 cm deep, but any you have around that size would be fine.)
Smooth the top of the mash and cover with the mini marshmallows.
Bake in the hot oven for 10 to 15 minutes, by which time the marshmallows will have coloured on top and melted together to look like a rippled, bronzed duvet.
NOTE: If you want to roast the sweet potatoes long in advance, it might make sense to make the purée and then heat it up in a saucepan before adding the marshmallows and blitzing it in the hot oven to brown.