Peel and cut the sweet potatoes into large chunks and place them into a large sauce pan. Cover them with with water and bring it to a boil.

Boil they sweet potatoes until they are fork tender and easily pierced with a fork. They should fall apart easily.

Drain the sweet potatoes and add them back to the sauce pan. Mash them with a potato masher. Add the rest of the sweet potato filling ingredients: butter, brown sugar, salt, cinnamon, vanilla extract, and nutmeg and mix until well combined.

In a separate bowl, make the pecan topping. Stir together the melted butter, brown sugar, all purpose flour, salt, and pecan pieces until combined.

Grease a 9 by 13 baking dish with cooking spray.

Add the sweet potato filling and spread out evenly.

Spread the pecan topping evenly over the top of the sweet potatoes.

Add the mini marshmallows over the top in an even layer.

Bake the sweet potato casserole at 350 degrees for 25-35 minutes, or until bubbly and the marshmallows are golden brown on top. Watch closely towards the end to get the perfect golden brown without burning.