Preheat oven to 375f and set the rack to the middle level.
To save time, pierce the sweet potatoes and microwave until soft (approximately 10-12 minutes). Check for doneness by inserting a knife through the potatoes, making sure the knife goes completely through with no resistance. Alternatively, the potatoes can be baked for 45-60 minutes in the oven at 375f or until soft.
Discard the potato skins and remove any fibrous material on their ends. Roughly chop the potatoes and add them to a large bowl.
Add the melted butter, milk, salt, cinnamon, vanilla extract, and half of the brown sugar to the potatoes. Beat until smooth with an electric mixer then taste test. Adjust any cinnamon or brown sugar to taste then add the eggs and mix until smooth and fully incorporated.
Hand mix half of the pecans into the bowl.
Spread the mixture into a 9 by 13 baking dish and top with the marshmallows, remaining brown sugar, and remaining chopped pecans.
Bake for 20-25 minutes or until golden on top. Enjoy!