Place the peeled and cubed sweet potatoes in a large pot and cover with cold water by about an inch. Add a pinch of salt.
Bring to a boil over high heat, then reduce to a simmer. Cook for 15-20 minutes until the potatoes are very tender and can be easily pierced with a fork.
Drain the potatoes well and return them to the pot or place them in a large mixing bowl.
Mash the potatoes using a potato masher or ricer until smooth.
Add the melted butter, maple syrup, warm heavy cream, cinnamon, nutmeg, vanilla extract, and salt. Stir until creamy and fully combined.
Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish.
Spread the mashed sweet potato mixture evenly into the prepared baking dish.
Sprinkle the chopped pecans over the top, followed by an even layer of mini marshmallows.
Bake for 15-20 minutes, or until the sweet potatoes are heated through and the marshmallows are puffed and golden brown. Serve warm.