Preheat the oven to 350 degrees F and arrange the oven rack in the center position.
Grease a large 9x13-inch casserole dish or baking dish. Set aside.
Thoroughly wash, peel, and chop the sweet potatoes into 1-inch cubes. Transfer to a large pot or Dutch oven and cover with cold water. Bring to a boil, cover, and reduce heat to a simmer. Allow potatoes to cook for approximately 15 minutes, or until tender when pierced with a fork.
As the potatoes cook, mix together the brown sugar, eggs, milk, butter, vanilla extract, ground cinnamon, and salt. Set aside.
Drain the cooked sweet potatoes in a large colander and transfer to a large bowl. Use a potato masher to mash the sweet potatoes until smooth.
Add the brown sugar and egg mixture to the mashed sweet potatoes. Mix thoroughly to combine.
Transfer the mashed sweet potato mixture to your prepared casserole dish, using a rubber spatula to spread it into an even layer.
In a separate bowl, combine the toasted pecans, all-purpose flour, brown sugar, melted butter, and a pinch of salt.
Sprinkle half of the prepared pecan topping over the top of your sweet potatoes followed by approximately 2 cups marshmallows, and finally, the last of the pecan topping.
Transfer to the oven and bake for 25-35 minutes, or until the tops of your marshmallows are golden brown and the potatoes are heated through.