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Peel and cut sweet potatoes into large chunks. Place in large sauce pan and cover with water. Bring to a boil and cook for 15 to 20 minutes until fork tender and falling apart easily.
Drain sweet potatoes and return to sauce pan. Add the stick of butter and mash with potato masher. The butter will melt from the heat.
Add brown sugar, salt, cinnamon, vanilla extract, and nutmeg to mashed sweet potatoes. Mix until well combined and smooth.
In separate bowl, make pecan topping by stirring together melted butter, brown sugar, all purpose flour, salt, and finely chopped pecan pieces until combined and crumbly.
Preheat oven to 350°F. Grease a 9×13 inch baking dish with cooking spray.
Spread sweet potato filling evenly in baking dish. Top with pecan mixture in even layer, covering entire surface.
Add mini marshmallows over pecan topping in even layer, covering completely.
Bake for 25 to 35 minutes until bubbly and marshmallows are golden brown on top. Watch closely towards the end to prevent burning. Let rest 5 to 10 minutes before serving.

Make Ahead: Prepare sweet potato filling up to 2 days ahead. Store in airtight container in refrigerator. Assemble with toppings before baking.

Pecan Substitute: Walnuts, hazelnuts, or sliced almonds can replace pecans for different flavor.

Without Marshmallows: Double the pecan topping and skip marshmallows for more sophisticated version.

Storage: Store leftovers in airtight container for 4 to 5 days. Reheat in oven at 350°F or microwave until heated through.