Preheat the oven to 350 degrees. Line a sheet pan with foil and coat with cooking spray. Prick the potatoes with a fork and place them on the pan.
Bake the potatoes until tender, 45 minutes to an hour.
When the potatoes are cool enough to handle, cut them open, scoop out the insides and place the potatoes in a large bowl.
Add the 4 tablespoons of butter, milk, brown sugar, maple syrup, cinnamon, egg and vanilla to the bowl.
Use a potato masher or hand mixer to mash or beat the potatoes until mostly smooth.
Grease a 9 inch square pan or 2 quart baking dish with cooking spray.
Spread the sweet potato mixture evenly in the pan.
In another bowl, mix together the brown sugar, flour, melted butter and pecans until crumbly. Sprinkle the crumb topping over the sweet potatoes.
Bake for 25-30 minutes or until topping has lightly browned.
Remove from oven, sprinkle the marshmallows over the top then return to the oven.
Bake for an additional 3-5 or until marshmallows are just melted.