Mix flour, salt, ground cinnamon, butter and dark brown sugar with a fork until a desired crumble texture is achieved. Add more sugar if you want finer crumbs.
Sweet Potatoes
Bring large pot with water to boil over high heat. Add sweet potato pieces and cook until fork tender, about 15 to 20 minutes.
Drain potatoes and place them back into pot or large bowl. Mash potatoes with a potato masher or blend with an electric mixer, until mostly smooth.
Add butter, brown sugar, salt, vanilla extract, nutmeg and cinnamon and using a potato masher, mash until combined.
Add heavy cream and combine for another minute until well incorporated.
Transfer sweet-potato mixture to a 2-quart baking dish. Top with 2 cups of mini marshmallows and set aside.
Preheat oven to 375 F.
Final Assembly
Top the sweet potato mixture and marshmallows with the prepared crumble topping.
Bake at 375F until center is warmed through and marshmallows and topping is lightly browned, 30 minutes.