Place the potatoes in a pot with cold water. Bring to a boil then cover and reduce heat to steam. Simmer for 15-25 min or until potatoes are tender enough to mash. Drain, let cool, transfer to a bowl. Mash sweet potato mixture with a potato masher.
Add brown sugar, butter, vanilla extract, cinnamon, nutmeg, salt, and 1/4 cup of pecans (save the rest for later). Mix until well combined. Add the potato mixture evenly. Sprinkle with remaining pecans and mini marshmallows, if using, in an even layer.
Bake at 375° for 15 minutes or until marshmallows are toasted. Remove from oven. Serve and enjoy!