Preheat the oven to 425°F. Start by placing your sweet potatoes on a baking sheet and place in the oven. Roast potatoes for about 50-60 minutes or until fork tender. Test this by piercing a fork through the thickest part of the sweet potato. Once tender, remove from the oven and set aside to cool. Once cooled, peel off the skin, carefully, and place potatoes in a large bowl. Using a hand mixer, blender, or potato masher, mash the potatoes down.
Whisk in sugar, brown sugar, cinnamon, egg, cream/milk, butter, vanilla extract, and salt until well combined. Be sure that the mixture is whisked until nice and smooth. Taste and adjust sweetness, as needed. If too thick, add more milk or cream. Next, pour your potatoes into an ungreased baking dish of your choice.
In a separate bowl, using a fork, combine your brown sugar, flour, cinnamon, and salt. Add in your softened butter and blend it into the mixture using your hands until you form a crumbled/wet sand texture. Place this on top of the sweet potato mixture in a baking dish and bake covered for 10-15 minutes.
Next, remove from the oven, top, and completely cover with marshmallows. Place back in the oven, uncovered, until golden brown on top. Remove and allow to cool slightly. Serve and Enjoy!
Notes
To save some time poke holes in the potatoes and wrap a damp paper towel around each one. Place in the microwave for 15-20 minutes.