Preheat the oven to 400 degrees F.
Wash the sweet potatoes, scrubbing them well to remove any dirt. With a fork, prick the sweet potatoes in a couple of spots and place them on a sheet pan. Bake for about 45 minutes, or until a knife inserted in the center goes in easily.
In a large bowl, mix the butter, brown sugar, and flour together until it's crumbly-looking. Add the cinnamon, salt, pecans, and marshmallows; fold the streusel topping together to combine.
Slice the sweet potatoes lengthwise down the center and push the ends towards the middle so it opens up. Stuff the sweet potatoes generously with the streusel topping and return to the oven. Bake for another 20 minutes, or until the topping is bubbly and brown.