Preheat oven to 400°F (200°C). Wash and dry sweet potatoes, then pierce them several times with a fork. Place them directly on the oven rack and roast for 45-60 minutes, or until they are easily pierced with a fork. Let them cool slightly before handling.
Once cool enough to handle, peel the sweet potatoes and place them in a large bowl. Use a potato masher or an electric mixer to mash them until smooth. Don’t overmix.
In the bowl with the mashed sweet potatoes, add the melted butter, brown sugar, maple syrup, cinnamon, nutmeg, ginger, vanilla extract, and salt. Mix well until everything is evenly combined. Taste and adjust the seasonings as needed.
Pour the sweet potato mixture into a prepared 9×13 inch baking dish and spread it evenly.
Sprinkle the mini marshmallows generously over the top of the sweet potato mixture, making sure to cover the entire surface.
Bake in a preheated oven at 350°F (175°C) for 20-25 minutes, or until the marshmallows are golden brown and slightly toasted. Keep a close eye on them to prevent burning. If the marshmallows start to brown too quickly, you can tent the casserole with foil.
Let the casserole rest for 10-15 minutes before serving.
