Boil sweet potatoes for about 20-25 minutes until they are soft (and can be pierced with a fork). Drain.
Preheat the oven to 375 degrees.
Mash sweet potatoes in a large bowl to desired consistency. (I like mine with a little texture, decide how smooth you want them)
Add in the egg, salt, vanilla, brown sugar, white sugar, melted butter and stir to combine.
Pour into a 9x13 in baking dish.
In a medium sized bowl, combine the brown sugar, flour, butter and chopped pecans and stir well.
Sprinkle mixture completely over the top of the sweet potatoes.
Bake uncovered for 20 minutes, or until the pecans are carmelized.
Spread marshmallows evenly over the top of the sweet potatoes.
Bake for another 3-5 minutes or until the marshmallows are puffy and golden brown.