Preheat the oven to 350°F (175°C).
Prepare the sweet potatoes by peeling and chopping them into chunks.
Boil the sweet potato chunks in a large pot until tender (about 15-20 minutes).
Drain the sweet potatoes and return them to the pot.
Add maple syrup, brown sugar, melted butter, vanilla extract, cinnamon, nutmeg, and salt to the pot.
Mash the sweet potato mixture until smooth.
Transfer the mixture to a greased baking dish.
If using, spread mini marshmallows over the top of the casserole.
Bake in the preheated oven for 25-30 minutes, or until heated through and marshmallows are golden brown.
Remove from oven and allow it to cool slightly before serving.
