For the Streusel Topping:
Preheat oven to 350 degrees. Clean sweet potatoes and boil or bake in the oven until tender. Cool, peel and mash the sweet potatoes. Stir in granulated sugar, mulling spices, butter, vanilla, milk and eggs. Mix well. Pour mixture into a 9 x 13 casserole dish for serving.
For the topping: Mix brown sugar, flour, nuts and butter together and spread evenly over sweet potato mixture. Cook for 30 minutes until topping has browned. If using mini marshmallows, add them the final 10 minutes of baking. Remove from oven and serve.