Place the sweet potatoes into a large pot and cover with water. Bring to a boil, reduce to a simmer, and let simmer for 15-20 minutes or until very tender.
Drain and set aside to cool for 5 minutes.
Transfer the sweet potatoes to a large bowl and mash with a potato masher until smooth.
Mix sweet potatoes with sugar, maple syrup, butter, cinnamon, vanilla extract, nutmeg, and salt until well combined. Optional: Add ½ cup of chopped pecans into mixture.
Spoon the sweet potato mixture into a lightly greased 9x13 inch pan. Spread in an even layer.
Sprinkle the pecans on top.
Bake for 30-40 minutes or until bubbly.
Top with an even layer of marshmallows. Turn oven to broil and return pan to oven. Cook for 1 minute until marshmallows have turned a golden brown (keep a close eye to avoid burning).