Peel and dice sweet potatoes. Cook in a pot with water until soft.
Preheat oven to 350 degrees F.
Once potatoes are cooked, drain and mash. Add butter and mix until melted.
In a small mixing bowl, whisk together eggs, brown sugar, vanilla and salt. Add to mashed sweet potatoes and mix well. Spread potato mixture in prepared casserole dish. Set aside.
In another mixing bowl, mix together sugar, flour, salt and nuts. Add melted butter and stir with a fork, until it forms crumbly dough.
Sprinkle dough in desired pattern over the mashed potatoes.
Bake casserole for 25 minutes. Add mini marshmallows. Place dish back in the oven and bake until the marshmallows are toasted, about 3 to 5 minutes.