Preheat oven to 450 degrees.
Melt butter in a large dutch oven over medium heat.
Add the sweet potatoes brown sugar, water, salt and pepper. Bring to a simmer, then cover and reduce heat to medium low.
Cook, stirring often, until the sweet potatoes are tender, about 45-60 minutes.
When potatoes are tender, remove the lid and bring the sauce to a rapid simmer over medium high heat.
Continue to simmer until the brown sugar mixture has reduced to a glaze, about 5 minutes.
Meanwhile, mix together all the remaining ingredients in a medium bowl. Set aside.
Pour the potato mixture into a 9×13 baking dish.
Spread the topping over the potatoes and bake until marshmallows are golden brown, about 10-15 minutes.