Rinse and scrub the sweet potatoes (leave them whole and unpeeled) and then either 1) Boil: Pierce them each a couple of times with a fork or sharp knife, then add them to a large pot of water. Boil until very tender, then drain them. Or, 2) Bake: Pierce them all over, microwave for 6 minutes, and then roast them at 425F for 20 minutes or until tender.
Preheat the oven to 375F and move the rack to the middle position. Grease a 9x13 baking dish.
Once the sweet potatoes are cool enough to handle, peel them and mash them in a large bowl with the brown sugar, butter, cream, eggs, vanilla, cinnamon, nutmeg, and salt. Ensure that they're well mashed and that the other ingredients are distributed evenly (e.g. no clumps of cinnamon). If the potatoes aren't mashing as smoothly as you like or you want them super smooth, you can use an immersion blender to purée them once you've manually mashed them as much as you can.
Pour the sweet potato mixture into the greased baking dish. Spread it in an even layer.
Top the sweet potatoes with the chopped pecans and the mini marshmallows.
Bake for 20 minutes or until the marshmallows are nice and golden. Watch it carefully towards the end so they don't burn (some ovens are hotter than others).