Preheat your oven to 375°F and grease a 9x13-inch baking dish with butter or cooking spray.
Arrange the sweet potato cubes in an even layer in the prepared baking dish.
In a medium bowl, whisk together the melted butter, brown sugar, maple syrup, cinnamon, vanilla, and salt until well combined.
Pour the butter mixture evenly over the sweet potatoes, then toss gently to coat everything.
Cover the dish tightly with aluminum foil and bake for 30 minutes.
Remove the foil and continue baking for another 15 minutes, stirring once halfway through, until the potatoes are tender and the sauce is bubbly.
If using marshmallows, sprinkle them over the top and broil for 2-3 minutes until golden brown—watch carefully so they don't burn!