Preheat the oven to 375 degrees. Spray a 9-inch by 13-inch baking dish with non-stick cooking spray.
Place the sweet potato cubes into a large pot with 2 cups of water. Bring to a boil, then reduce heat and cover. Simmer potatoes for 20-25 minutes or until softened through. Drain any excess liquid and transfer to a bowl to cool.
Once potatoes have cooled, mash with a potato masher or fork until desired consistency is achieved.
Stir in butter, vanilla extract, cinnamon, 3/4 cup of brown sugar and ¼ cup of the chopped pecans until well combined.
Spoon the sweet potato mixture into the prepared baking dish.
Toss the remaining 1/4 cup brown sugar and 3/4 cup pecans together in a small bowl. Sprinkle the mixture evenly on top of the potatoes. Top with the mini marshmallows.
Bake in the oven for 15 minutes, until marshmallows are slightly golden and toasted on top. Serve warm!
Make it AheadThis casserole can be prepared the night before and refrigerated. Chill, covered, without the topping. Remove from refrigerator 30 minutes before baking. Bake just the sweet potato mixture for 20 minutes at 375. Add the topping and bake 15 more minutes until the marshmallows are toasted.