Prep. Preheat your oven to 350°F (175°C). Place the prepared sweet potatoes in a large mixing bowl and set aside.
Make sugar syrup. In a large skillet, melt the butter over medium heat. Add the sugars, cinnamon, nutmeg, black pepper and salt to the melted butter. Stir until well combined and the sugar has dissolved.
Toss potatoes in syrup. Pour the sugar mixture over the sweet potatoes and toss together until the sweet potatoes are evenly coated in the sugar mixture.
Bake to soften potatoes. Transfer the coated sweet potatoes and any remaining sugar mixture on the bottom of the bowl into a baking dish or cast iron pan. Bake, covered with foil, for 50-60 minutes or until the sweet potatoes are tender and the sauce has thickened.
Add marshmallows and bake briefly again. Remove the baking dish from the oven and evenly sprinkle the mini marshmallows over the sweet potatoes. Return the baking dish to the oven and bake for an additional 5-10 minutes, or until the marshmallows are golden brown and toasted.