I goofed with this, and overbaked it so the topping actually melted down into it and it was swimming in goo. So we had sweet potato in sauce rather than the soft and cludgy bake that I remembered.
The basic idea is to layer cooked, peeled sweet potato with butter and sugary stuff, a bit of salt, maybe some rum, or nutmeg, and bake at about Gas Mark 5 for about half an hour until it's all soft, and top with marshmallow and bake for 10 – 15 minutes more so it gets a sweet crust.
I thought steaming the potatoes to cook them and peeling them afterwards would be easiest, it wasn't! Buying pre-prepared or peeling raw would be best. Don't worry if the cooked flesh loses its colour, cooking it again with the sugar will bring it back.
Next time I'm going to go with the alternative, which is to mash the potatoes with the seasonings, heat that, and flash grill the topping. It'll be easier to control the texture and sweetness.
Maple syrup is a good sweetener, but it creates that bit more liquid than brown sugar. I used salted butter, which I rarely do nowadays, and found I only needed to add a pinch of salt. Definitely a once a year treat, though.