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.
While potatoes are baking..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. Set aside until sweet potatoes are out of the oven.
Slice the sweet potatoes lengthwise down the center and push the ends towards the middle so it opens up. (Like you see a typical baked potato served at a restaurant). 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.