Preheat oven to 475°.
Prick or puncture the sweet potatoes with a fork and put them each on a piece of foil large enough to wrap them inches.
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, approximately 1-1 1/2 hours.
When the sweet potatoes are cool enough to handle, strip away the skin and for 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 for the marshmallows, and mix together to make the mash.
Spoon the sweet potato mash into a 13 x 9 inch baking dish; smooth the top of the mash and cover with the mini marshmallows.
Bake in the hot oven for 10-15 minutes by which time the marshmallows will have colored on top and melted together, like a rippled, bronzed duvet (watch carefully for overbrowning).